Labour has urged the Conservatives to follow its lead by revealing the names of all those who gave them loans before the last general election.
Loans were spent on the election
Environment Secretary Margaret Beckett has written to Tory leader David Cameron asking him to publish a full list of individuals and organisations.
Under pressure Labour has revealed the 12 people who loaned them almost £14m.
The Tories have promised to disclose future loans but say it would be unfair to backdate the new rules.
Some of their backers wanted to remain anonymous because they feared they would lose government contracts, said a party spokeswoman.
Betting millionaire Stuart Wheeler, who has given the Tories more than £5 million in recent years, said confidentiality agreements should not be breached.
"If promises have been made to a lender, they shouldn't be broken without the permission of the lender, unless the law is changed to require the party to reveal them," he said.
Tory backbench MP Roger Gale agreed that lenders should be asked if they are willing to have their names released.
Mr Gale accused Labour of trying to distract attention from their own "wholly unacceptable" fundraising tactics.
The Electoral Commission has written to the treasurers of all the main parties asking them to declare all loans which "are not on commercial terms", including those which "may be converted into a donation at a later stage".
They have until next Wednesday to respond.
Mrs Beckett's letter is a clear attempt to keep up the political pressure on the Conservatives, says BBC's political correspondent Mike Sergeant.
It echoed calls from Work and Pensions Secretary John Hutton for the Tories to "come clean" about its past lenders.
"It is time for David Cameron to match his words on openness with actions to declare all Tory loans," he said.
His party named 12 backers who lent it almost £14m before the last general election after it emerged four of them had been nominated for peerages.
Labour's loans were made with interest rates of between 1% and 3% above the base rate.
Labour's then General Secretary Matt Carter sent letters to donors after the loans had been made.
The purpose was to "set out the legal terms upon which the Labour Party received the loans so as to ensure it was done in a fair and legal way, strictly within all legal guidelines," a Labour spokesman said.
Following the disclosure of the loans, Mr Blair announced plans to withdraw as prime minister from the process of nominations for honours. This did not affect his ability to create peers.
An inquiry into party funding has been launched by the influential constitutional affairs committee, who will also interview Tony Blair's chief fundraiser Lord Levy.
He will also face questioning from the public administration committee.