Former Tory party treasurer Peter Cruddas claimed to have direct access to the Prime Minister on at least 13 occasions.
Labour has raced into a poll lead as the Tories deal with fall-out from the Budget and the cash-for-access controversy, a survey shows.
The Tories have released a list of party donors who attended a lunch with the PM, with Labour calling for an independent inquiry.
David Cameron has admitted he owes an apology to former Conservative party treasurer Peter Cruddas, who was made an "outcast" by the party after newspaper sleaze allegations.
Mr Cruddas was dismissed from the role after the Sunday Times reported that he was charging £250,000 to meet Mr Cameron but has since won a libel action against the newspaper.
High Court judge Mr Justice Tugendhat criticised the Prime Minister's response, saying he had subjected his former Conservative colleague to a "public humiliation".
Mr Cameron said he was "very sorry" about the treatment of Mr Cruddas, who claimed he was "cut off" by the party and "made to feel like an outcast" by the Prime Minister.
Mr Cameron said he was looking forward to meeting his former colleague later in the year.
He said: "I rather think I do owe him an apology."
This is significant because Downing Street had always insisted that Peter Cruddas was a minor figure - and here he is seen boasting about his closeness to the Prime Minister.
It also adds weight to the accusation that Downing Street hasn't come clean about the numbers of donors the Prime Minister met at Chequers.
Number 10 will insist this is insignificant - but the Labour Party will no doubt jump on this and say that Ed Miliband has been much more transparent about the donors and backers he's had dinner and meetings with.
"I feel that, as I work for the party," former Conservative Party Treasurer Peter Cruddas tells Sunday Times reporters, "my job is to get the donors in front of the Prime Minister."
He describes Chequers as "fantastic" and says of one dinner he attended, "I was sitting next to Sam [Cameron], because I sponsored the evening."
The Sunday Times has released new undercover footage of former Tory party Secretary Peter Cruddas discussing some of the many occasions he mixed with David Cameron and party donors at private events.
Asked whether he'd publish a full list of his dinners and meetings, Labour leader Ed Miliband said, "I'm very happy to be open and transparent about what we're doing."
"I think the stain on the character of the Government needs to be dealt with and I think only a full independent inquiry, done by the independent adviser on ministerial interests, will do," he added.
Labour have raced into a double-digit opinion poll lead over the Conservatives after the fall-out from the Budget and cash-for-access controversy, a ComRes survey for The Independent shows.
- Labour - 43%
- Conservatives - 33%
- Liberal Democrats - 11%
- Others - 13%
William Hague has told ITV1's The Agenda that donors do not influence him on his foreign policies. Mr Hague said that "if anyone ever influences me on any policy it's somebody that I meet at surgery or somebody I meet in Afghanistan or Iraq".
Mr Hague denied the existence of a 'policy committee' and said: "Peter Cruddas resigned because he said some silly things. Things that he acknowledges were not true, that there was a policy committee- there isn't such a committee that donors can feed into".