Tony Blair's fundraiser, Lord Levy, and his Lib Dem and Tory counterparts have been grilled by MPs looking at how political parties should be funded.
The peer is Tony Blair's chief fundraiser
The inquiry follows claims - denied by all involved - that peerages were given in return for donations and loans.
The three men gave evidence to the MPs behind closed doors to avoid damaging the prospects of any police action.
The Commons Constitutional Affairs Committee also met the officer heading a police probe into the claims.
It released a statement confirming several issues had been discussed with the peers.
These included "the challenge of raising funds for political parties in an era of diminished membership of political parties and the cost drivers for high levels of party spending in elections", it said.
The men were also asked for "their views on proposals by the Electoral Commission on spending caps and on the advantages and disadvantages of state funding of political parties".
Questions were also asked "about the relationship between local and national systems of fundraising", the statement added.
Scottish National Party MP Angus MacNeil, whose complaint sparked the police inquiry, had urged the committee not to interview any possible police witnesses.
He had written to committee chairman Alan Beith saying he was alarmed by their plans.
"I would urge you to follow the example of the public administration committee and refrain from doing anything that could jeopardise the integrity and evidential value of the present Scotland Yard inquiry," he said.
But the committee said it would go ahead with its hearing, in private, because it was not looking specifically at the issue of cash for honours.
Lord Levy has earned the media nickname "Lord Cashpoint" because of his fundraising efforts for Labour at recent elections.
The MPs also questioned Conservative Party treasurer Lord Marland and his Lib Dem counterpart Lord Razzall.
Scotland Yard's Deputy Assistant Commissioner, John Yates, has asked parliamentary committees to postpone interviewing potential witnesses until his team's inquiries are complete.
He met the committee after their session with Lord Levy and is likely to update MPs on the progress of his investigation.
The committee's statement emphasised that members had "taken fully into account the need to avoid any risk of prejudicing police inquiries" during their discussions with the Metropolitan Police.
All three main parties have denied any allegations of cash for honours.