Police handling of the cash-for-honours probe was "absolutely proper", the man leading the 16-month inquiry has said.
John Yates says he followed the evidence
Assistant Commissioner John Yates said he knew it had "not been a comfortable time" for those involved but said the inquiry had to be "meticulous".
Earlier the Crown Prosecution Service said no charges would be brought.
Mr Yates' boss, Met Police chief Sir Ian Blair, is to probe the handling of the inquiry amid concerns about its length and £1m cost.
He will produce a report for the Metropolitan Police Authority, (MPA) which oversees the force.
MPA chairman Len Duvall said: "I want to stress that I am not judging the rights or wrongs of the inquiry at this stage.
"But the public has the right to know that the police have carried out their duties appropriately, particularly where substantial resources were committed. People need to know that the investigation was fair, impartial and proportionate."
Although many of those questioned by police during the inquiry, including former prime minister Tony Blair, have said they are not criticising police, others have raised questions about the way it was conducted.
Over 16 months, 136 people were interviewed and four arrested during the investigation into claims peerages had been offered in return for loans to political parties - which later widened into whether there had been an attempted cover-up.
The inquiry began with a complaint from the Scottish Nationalist MP Angus MacNeil, and some Labour MPs have complained that police put so much time and effort into what they see as a "political stunt".
But in a statement on Friday, Mr Yates said the decision to launch an investigation followed "cross party" allegations and some from "several members of the public".
He also pointed to the original Sunday Times report and "comments from others in early March 2006 that the Labour Party had deliberately concealed a number of loans made to it at the time of the 2005 general election".
He said police had to go "where the evidence takes you" and if there had been allegations of an attempted cover-up the inquiry would have finished "much earlier".
'In the spotlight'
"I was clearly aware that any investigation into these matters would have serious consequences. The decision to investigate was not one that was entered into lightly," he said.
He added that the decision on whether to prosecute or not was a matter for the CPS "and those that advise them", while the investigation was his responsibility.
He added: "Some have been surprised by the intensity and the length of the police investigation and I recognise that it has not been a comfortable time for many of those who came under the investigative spotlight."
"However, with allegations such as these, which were of the utmost seriousness, the investigation had to be thorough and meticulous in every respect.
"It was absolutely proper therefore that when appropriate police used the full range of powers at their disposal to gather the available evidence or obtain the best possible evidence. Our job is to follow the evidence."
He said he would now provide the Metropolitan Police Authority with a report accounting for "both the cost and the outcome of what has been a high profile and resource intensive investigation".
Earlier, former Labour minister Lord Foulkes said it was "quite astonishing" that a lengthy and costly investigation had been launched "without any apparently prima facie evidence" and said questions "must be asked and answered".
Lord Levy criticised "damaging" leaks about the inquiry
Metropolitan Police Commissioner, Sir Ian Blair, has been ordered to produce a report into the way the honours investigation was carried out.
In a statement, he said: "From the start of the investigation I made public that I would not be involved in operational decisions as I felt this would be inappropriate given the regular meetings I have with cabinet members and the former prime minister on official business."
But he said he had been kept informed of the investigation's process and had "always been satisfied by the leadership and conduct of the investigation".
"Assistant Commissioner John Yates and every member of his team have carried out this complex and sensitive investigation with the utmost professionalism. "They all continue to have my full support and confidence."
Conservatives on the London Assembly have called on Sir Ian to appear before the next meeting of the MPA to answer questions on the inquiry.
Spokesman Richard Barnes said: "It is most disturbing that the police should spend up to £1m on an investigation which lasted nearly 16 months and ending with the CPS laying charges against no one."
He added: "We all need to be assured that the decisions reached were reached properly and without outside interference."