The Crown Prosecution Service has said no-one will face charges following the 16-month cash-for-honours investigation. Here is our guide to what happened.
What was the police inquiry all about?
Scotland Yard investigated claims that laws made in 1925 banning the sale of honours had been broken by political parties giving peerages in return for donations and loans. It also investigated whether a law made in 2000 which says that all donations of more than £5,000 must be declared, had been breached. During the investigation, the police also asked questions about alleged conspiracy to pervert the course of justice. All concerned in the inquiry denied wrongdoing.
What prompted the investigation?
It emerged last year that a number of large secret loans had been made to the Labour Party before the 2005 general election, and that some of those lenders had subsequently been nominated for peerages. Scottish National Party MP Angus MacNeil wrote to the Metropolitan Police asking them to investigate whether any laws had been broken. The investigation was later widened to cover the other main parties.
What is the latest development?
The CPS announced on Friday that no-one is to face charges.
How big has the investigation been?
It began last March and has seen 136 people interviewed. The main file handed over by the police to the Crown Prosecution Service was 216 pages long, and more than 6,300 documents have also been handed over.
Did the probe focus on Tony Blair?
Mr Blair became the first prime minister to be questioned by police in the course of an investigation. He was interviewed three times - in December, January and June - but not under caution, so he was being treated as a witness rather than a suspect.
What about his aides?
Mr Blair's chief fundraiser Lord Levy was arrested for a second time, on suspicion of conspiracy to pervert the course of justice, in January. Downing Street's director of government relations, Ruth Turner, was arrested on 19 January, 2007, on suspicion of offences under the Honours (Prevention of Abuses) Act. She was also questioned on suspicion of perverting the course of justice. Answering bail on 20 February she was again interviewed for about two hours before being bailed again, pending further inquiries. She was previously questioned by police in September, but not under caution. Both have always denied any wrongdoing.
What sparked a cover-up probe?
The BBC was told there was a document in which Ruth Turner expressed her concern that Lord Levy had put to her a version of events she believed to be untrue.
What did Lord Levy say?
Lord Levy said in a statement there had been a "regular stream of leaks to the media during this year-long investigation, all of which have presented a prejudiced and distorted view". This had "created a climate which does not allow for any fair assessment of the investigation", it added. A statement from his solicitor strongly denied any "wrongdoing whatsoever".
Who else was arrested during the inquiry?
Biotech boss Sir Christopher Evans, who lent Labour money, was arrested and interviewed. In April 2006 Des Smith, a head teacher who helped find sponsors for the government's flagship city academies programme, was arrested and questioned. Both denied any wrongdoing.
In February 2007 the Crown Prosecution Service said there would not be any charges against Mr Smith, due to "insufficient evidence".
What do those who were arrested and subsequently released without charge say?
Lord Levy, Ruth Turner and Sir Christopher Evans have all expressed relief and delight that the investigation is finally over.
There has been no direct criticism of the police, although Lord Levy attacked "damaging" leaks he said had occurred during the course of the investigation.
What about Tony Blair?
He has said the investigation had ended as he "always expected it would".
He added: "Much of what has been written and said about them [those involved] has been deeply unfair, and I am very pleased for all of them that it is now over.
"I want to make it clear that I level no criticism at the police."
What do the police say?
Assistant Commissioner John Yates, who headed up the investigation, said from the outset that his officers were simply doing their job.
He defended the decision to pursue the "challenging case", saying it had not been made lightly.
He stressed that the allegations had been very serious, adding: "It was absolutely proper therefore that, when appropriate, police used the full range of powers at their disposal to gather the available evidence or potential evidence."
He said the police had to go where the evidence took them but the investigation would have ended sooner if it had not been for allegations of a cover-up.
Met Police Commissioner Sir Ian Blair backed Mr Yates but said he would be producing a report on lessons that could be learned from the investigation.