The complaint was brought by the Scottish National party MP Angus McNeil after it emerged that four wealthy businessmen, who had lent the Labour party a total of £5m, were nominated by Tony Blair for peerages. All four of the peerages were later blocked by the House of Lords appointments commission.
April 13 2006: Des Smith is the first person arrested. Mr Smith, who until January was an adviser to the body that finds wealthy sponsors for the government's city academies, had allegedly suggested that backers of a flagship Labour schools policy could expect to be rewarded with honours. His lawyers later say he "categorically denies" the allegations.
July 12 2006: Lord Levy, Tony Blair's chief fundraiser, is arrested for the first time. He is later re-interviewed and re-bailed without charge and insists he is innocent.
July 13 2006: The Commons public administration committee calls for tighter checks on the propriety of all higher honours awards.
July 14 2006: It emerges that police officers have already questioned at least two government ministers. Former Labour party chairman Ian McCartney and the science minister, Lord Sainsbury (who has since stepped down), are among 48 people interviewed by this stage.
July 16 2006: Mr Blair acknowledges a "real problem" with the public's perception of the "cash-for-honours" row but says he believes that nobody in the Labour party has broken the rules.
August 23 2006: Political parties generally are urged to improve their disclosure of financial support after more than £300,000 of donations were declared late. The Electoral Commission says late reporting is "not acceptable".
September 20 2006: Sir Christopher Evans, the biotech mogul who made a £1m loan to Labour, is the third person to be arrested. He is bailed without charge. The next day he says he is "extremely shocked and dismayed" and insists: "I have done nothing wrong."
September 28 2006: It emerges that Downing Street aide Ruth Turner has been questioned under caution. She is believed to have been asked about emails and documents relating to the inquiry.
October 1 2006: Bob Edmiston, the leading Tory donor and car importer, is reported by the Times to have been questioned under caution.
October 17 2006: Voters want to see donations to political parties capped as part of reforms in the wake of recent scandals, says research by the Electoral Commission.
October 23 2006: Michael Howard, the former Conservative leader, says that he agreed to be interviewed by police investigating the "cash-for-honours" affair. He is not been arrested nor questioned under caution.
November 4 2006: Mr McNeil calls for Lord Goldsmith to distance himself from the inquiry following reports that he might be responsible for making the final decision over whether criminal charges are brought. Mr McNeil says that there is an "obvious conflict of interest" given Lord Goldsmith's political links to the prime minister, but the attorney general refuses to rule out having the final say.
November 6 2006: The prime minister refuses to comment on the row over Lord Goldsmith. Meanwhile, Scotland Yard and Downing Street will not comment on reports that Jonathan Powell, the Downing Street chief of staff, is to be interviewed under caution.
November 8 2006: It emerges that virtually all ministers who served in the cabinet in the run-up to the 2005 general election have been contacted by Scotland Yard, asking them to declare formally in writing what they knew about the loans.
They included such senior figures as Gordon Brown, John Prescott, and Ruth Kelly, but not Mr Blair. Alan Milburn, the former health secretary, also reveals that he has been questioned over the affair.
It also emerges that Patricia Hewitt was questioned.
November 10 2006: Lord Sainsbury resigns as science minister but says that his decision is for personal reasons and has nothing to do with the inquiry.
November 12 2006: Mr Prescott, the deputy prime minister, declares himself satisfied with the police's conduct as Downing Street rejects reports that officials at No 10 have complained about how it has been handled.
November 16 2006: In a letter updating MPs, the Met's deputy assistant commissioner, John Yates, who is leading the inquiry, says the investigation has turned up "significant and valuable material" and that "considerable progress continues to be made".
He reveals that his Scotland Yard team has conducted 90 interviews so far - 35 from the Labour party, 29 from the Conservatives, four from the Liberal Democrats, and 22 people not linked to any particular party.
Mr Yates tells the House of Commons public administration select committee that he hopes to be able to send a file of evidence to the CPS in January. It also emerges that former Labour party chair Hazel Blears met Mr Yates. Her spokesman said it was a meeting "to keep her up to date" with the matter.
November 18 2006: Mr Yates, it emerges, has been asked by the committee to beef up his letter to them detailing the progress of his inquiry after an initial submission was considered "too cursory". It also becomes clear the letter was published at the request of the MPs.
November 20 2006: Lord Goldsmith says he does not expect to be interviewed by detectives as he had not been involved in any Labour party fundraising. "I am not going to be involved in the process - with fundraising issues - that they are concerned with, so I can't see why I should be," he says.
November 21: It is reported that Des Smith, the first person arrested in the investigation, has been given his laptop computer back by police. Labour also complains to the BBC after it emerges that journalists were offered £100 "bounty" payments for stories on the cash-for-honours affair.
November 22 2006: Ms Hewitt, the health secretary, has a "brief" interview with the police, talking to them as a witness. She is believed to have faced questioning over donations made to her constituency party in Leicester West by curry tycoon Sir Gulam Noon - one of those who later loaned the party money and was blocked for a peerage.
December 14 2006: Downing Street announces that Mr Blair has been questioned by police. He was not under caution.
January 19 2007: Ms Turner is arrested under the Honours (Prevention of Abuses) Act 1925, and also on suspicion of perverting the course of justice.
January 19 2007: MPs on the constitutional affairs committee reveal evidence showing that the attorney general, Lord Goldsmith, overruled the lord chancellor, Lord Falconer, in refusing to stand aside from the cash-for-peerages probe.
January 30: Lord Levy is arrested for a second time, this time on suspicion of conspiring to pervert the course of justice.
February 1: It emerges that Mr Blair was interviewed by police for a second time on January 26. The prime minister was questioned for less than an hour by officers, but the police requested a news blackout, according to Downing Street.
February: Des Smith will not face charges, the Crown Prosecution Service says.
February 20: Ms Turner is questioned for a third time by police. She is released on bail, pending further inquiries.
March 3: The attorney general obtains an injunction against the BBC to stop it broadcasting a report about the cash-for-honours investigation.
March 5: The injunction against the BBC is amended so that the broadcaster can report that it related to a document written by Ms Turner to Mr Powell about Lord Levy.
March 6: The Guardian defies the attorney general to report that detectives were investigating whether Lord Levy urged Ms Turner to shape the evidence she gave to Scotland Yard.
The BBC injunction is partially lifted, and the broadcaster reports that Ms Turner had written that she was worried that Lord Levy had put to her a description of "his role in drawing up the honours list which she believed to be untrue".
March 12 2007: The final ban on a BBC story which had been the subject of an injunction by the attorney general is lifted, allowing the broadcaster to claim that Ms Turner told Mr Powell that Lord Levy had asked her to lie for him during the cash-for-honours investigation. Lord Levy denies all wrongdoing.
March 20: The Met police hand over their report to the Crown Prosecution Service. It is 216 pages long with 6,300 supporting documents. The investigation has reportedly cost £800,000.
May 23: Lord Levy announces he will step down as the PM's special envoy to the Middle East when Mr Blair leaves Downing Street. There is widespread criticism that his later leaving party at the FCO is funded by the taxpayer.
June 28: The day after Tony Blair steps down as prime minister it emerges that he has been questioned for a third time by the Met police, again as a witness. The interview is believed to have taken place three weeks previously.
July 7: The CPS announces it has enough evidence to make a decision on charges.
July 19: Sources say the CPS will not bring charges against any of the three arrested.
July 20: CPS confirms it has insufficient evidence to charge anyone in the case.
October 9: CPS says there is "insufficient evidence" to charge anyone in relation to cash-for-honours allegations against the Conservative party.
October 11: The House of Commons public administration committee relaunches its inquiry into the allegations, which was suspended while the police investigation took place.