Tony Blair has said he is "very pleased" that the cash-for-honours inquiry is over, saying those involved had been through a "traumatic time".
Tony Blair was the first sitting PM to be quizzed by police
The former PM was responding to the decision not to bring any charges following a 16-month police inquiry.
Assistant Commissioner John Yates said the allegations had been serious and the way police had handled the inquiry had been "absolutely proper".
Mr Blair said the inquiry ended "as I always expected it would".
Biotech tycoon and Labour donor Sir Christopher Evans, who was arrested at one point during the investigation and released without charge, said he was "glad that it is all over".
Sir Christopher, who was among a group of wealthy backers to lend Labour a total of £14 million to bankroll its 2005 general election campaign said there were "never any conditions on my support for the party".
He added: "My firm belief that I had done nothing wrong or illegal has been completely borne out by today's decision."
Police interviewed 136 people, including Mr Blair and former Tory leader Michael Howard, during the inquiry.
The investigation into claims that people were nominated for peerages in return for political loans was prompted by a complaint from the Scottish Nationalist MP Angus MacNeil.
Mr Blair said: "Those involved have been through a terrible, even traumatic time. Much of what has been written and said about them 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. They were put in an invidious position by the SNP complaint and had a very difficult task to perform. "
Metropolitan Police Commissioner Sir Ian Blair has launched an internal investigation into the £1m cost and outcome of the honours probe.
Assistant Commissioner Yates, who led the inquiry, 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."
Lord Levy said the news was a "great relief"
Earlier the Crown Prosecution Service confirmed that no charges would be brought and, in an unusual move, released a document explaining its reasons.
It says: "For a case to proceed, the prosecution must have a realistic prospect of being able to prove that the two people agreed that the gift, etc, was in exchange for an honour."
'Delighted and relieved'
But it concludes there was "no direct evidence of any such agreement between any two people subject of this investigation".
And it adds each of the donors at the centre of the investigation "was a credible candidate for a peerage, irrespective of any financial assistance that they had given, or might give, to the Labour Party".
The CPS also stresses "political questions have played no part in its analysis of this case".
Labour's former chief fundraiser Lord Levy, one of four people arrested, said the decision was a "great relief" and thanked his family, friends and legal team for their support.
But he said, while he did not want to comment on the police investigation, he had been "disappointed by constant leaks to media which have been misleading, factually inaccurate and personally damaging to me".
Head of the CPS Special Crime Division Carmen Dowd paid tribute to the "diligence and professionalism" of the police officers involved but said there had been "insufficient evidence to provide a realistic prospect of conviction against any individual for any offence in relation to this matter".
Prime Minister Gordon Brown has already said police were right to investigate the "very serious allegations" but he hoped the CPS statement would bring an end to "months of speculation".
But some Labour MPs have criticised the police decision to put so much time and effort into the inquiry, prompted by the SNP complaint, which they saw as a "political vendetta".
Four people were arrested during the inquiry - Lord Levy, Mr Blair's director of government relations Ruth Turner, former head teacher Des Smith and Labour donor Sir Christopher Evans.
Ms Turner said the CPS announcement was "an enormous relief" following a "very stressful time for me and my family".
She added: "I know that, however difficult for me personally, the police had an obligation to investigate these allegations thoroughly, and I co-operated with them fully. I am now looking forward to getting on with my life."
Mr Blair, who was questioned three times as a potential witness, became the first sitting prime minister to be interviewed by police in the course of a criminal inquiry.