The cash-for-peerages investigation lasted 16 months and saw police interview 136 people. Here is a guide to the main players involved:
Former prime minister and leader of the Labour Party Tony Blair denied accusations of offering peerages for cash after four people he nominated for honours were found to have made substantial loans to the party ahead of the last election without the knowledge of Labour's treasurer and other elected officials. Donors' names already had to be published and Mr Blair promised to bring in similar rules to cover people who lend political parties money. In December 2006, he became the first serving prime minister to be questioned by police as part of a criminal investigation. He was interviewed again in late January and in June 2007. Downing Street stressed that he was not cautioned, which means he was being treated as a witness rather than a suspect.
John Yates is assistant commissioner at Scotland Yard and the man who led the cash-for-peerages probe. He is no stranger to controversy. He headed an investigation into internal police corruption in the 1990s and led the inquiry that prompted the successful bringing of perjury charges against Lord Archer.
Labour's fundraising chief and close friend of Mr Blair, Lord Levy was dubbed "Lord Cashpoint" by the press. The former music business impresario raised nearly £14m in loans from wealthy individuals ahead of the 2005 election. Now believes parties should be funded by the state. Arrested on suspicion of possible infringements of honours and election laws, and, six months later, arrested again on suspicion of conspiracy to pervert the course of justice. Has always denied any wrongdoing.
Former director of government relations, Ruth Turner was the first Downing Street official to be arrested. She was questioned about alleged honours offences and on suspicion of perverting the course of justice. She was released without charge and has always denied any wrongdoing. Known as Tony Blair's "gatekeeper" she was a close aide.
Director of political operations in the Tony Blair's office, John McTernan helped draw up names for honours. Was quizzed twice by police, once under caution. Known as a political fixer, Mr McTernan helped to run Labour's election campaign in Scotland and is thought to have co-ordinated Mr Blair's exit strategy from Number 10.
Tony Blair's chief of staff Jonathan Powell is believed to have been interviewed twice by police, the second time under caution. A member of Tony Blair's inner circle since his election in 1997, the former diplomat and brother of Margaret Thatcher's foreign policy adviser Charles Powell, was thought to be Mr Blair's closest confidant when he was in Downing Street.
Former Labour Party chairman Ian McCartney signed papers confirming the party had received money from lenders while in hospital recovering from a triple heart by-pass operation. Said "every penny" of the loans was spent on getting Labour MPs re-elected.
Former Labour general secretary Matt Carter is thought to have come up with the secret loan plan with Lord Levy and Mr Blair after the extent of the party's cash crisis became apparent ahead of the last election.
Des Smith was the first person to be arrested during the cash for peerages row. The head teacher helped find sponsors for the government's flagship city academies programme. He allegedly told an undercover reporter that people like a donor would be recommended for honours, but he denied any wrongdoing. The Crown Prosecution Service announced on 6 February that there was "insufficient evidence" to charge Mr Smith with an offence under the Honours (Prevention of Abuses) Act 1925.
SIR CHRISTOPHER EVANS
Biotech chief Sir Christopher Evans lent money to Labour before the election and said he was shocked when he was arrested and questioned in the police inquiry. Sir Christopher was knighted in 2001 for his services to the biotechnology sector, after earlier being given an OBE in 1995. He runs a venture capital company called Merlin Biosciences.
Labour Party treasurer Jack Dromey is also deputy general secretary of the Transport and General Workers Union and member of Labour's ruling national executive. Sparked an internal probe into Labour's finances when he revealed he had been kept in the dark about loans from wealthy individuals. Accused Mr Blair of lacking respect for the party. He is married to minister and Labour deputy leader Harriet Harman.
Former permanent secretary at the Department of Culture, Media and Sport, Hayden Phillips carried out a review of how parties are financed, with a brief to look in detail at the possibility of state funding.
Scottish National Party MP Angus MacNeil first made the complaint which prompted the police inquiry into cash-for-honours allegations. Mr MacNeil was repeatedly highly critical of Tony Blair's government, calling Mr Blair "the Arthur Daley of Westminster politics".
Labour revealed it had secretly borrowed £14m from party lenders ahead of the last election, while the Conservatives borrowed £16m from 13 wealthy backers.