The Labour Party did not arrange any loans in the three months after the "cash-for-peerages" affair emerged, Electoral Commissions figures show.
Political loans will have to be declared from next month
Labour got £3.4m in donations. Although it took out no new loans it said £28.2m was still owed on earlier loans.
The Conservatives were loaned £2.5m over the same period - April to June this year - and got £5.9m in donations.
The Lib Dems declared donations worth £782,780.The party also said it owes £584,339 on outstanding loans.
The cash-for-peerages row was prompted in March by the revelation that some people nominated for peerages had secretly lent large sums to Labour.
The party, which denies any wrong-doing, did not have to declare loans given on commercial terms - exploiting, as the Conservatives also did, a loophole in rules which mean anyone donating more than £5,000 has to be declared.
As a result of the large loans coming to light the rules have been changed so all large loans have to be declared from next month.
The parties volunteered to declare large loans this time around.
Labour and the Lib Dems declared all their outstanding loans, but the Conservatives declared loans received in the second quarter of 2006, rather than all the cash it owes to donors.
"The Labour Party has declared all its current loans as will be required by legislation which will come into force this September," said Labour's general secretary Peter Watt.
"It appears that the Tories have only declared loans taken out in the last quarter. In addition to this they still have not declared who their overseas lender was.
"If the Labour Party was to declare in the same way as the Conservatives, we would have had no loans to declare for this quarter."
The Liberal Democrats also criticised the Conservatives for only disclosing loans made to them since April.
Lib Dem chairman Lord Rennard said: "Public confidence in politics is likely to suffer further unless all political parties are open and transparent.
"It is a matter of deep regret that the Conservatives have only decided to report loans made to them since 1st April.
"They should immediately file a report to the Electoral Commission, ensuring that all their outstanding loans are a matter of public record."
The Conservatives said they had released details of their outstanding loans, which amounted to £19m, earlier this year. And they accused Labour being too reliant on trade union funding, which accounted for 74% of its income in the three months to 30 June - and of blocking Tory plans for a cap on political donations.
Tory Chairman Francis Maude said: "Labour is now almost entirely dependent on the unions for funding. In return, they're getting pet policies and bungs with taxpayers' money.
"This sort of cronyism undermines our entire democratic process."
The Electoral Commission said the Tories had done nothing wrong by not releasing details of all of its donations.
Commission chairman Sam Younger said: "I don't think there is any evidence any of the main parties are being dishonest. The voluntary code we put out was precisely that - voluntary."
The parties would have to declare all donations by law from next month, he said.
But, he added, all the parties were still being too secretive about their finances and should make more of an effort to supply accounts information.
He said he was "disappointed" some of the smaller parties had chosen not to declare loans made to them.
But he also criticised Conservatives over the late reporting of some donations - mainly from local Conservative Associations.
Mr Younger said: "What we are frustrated by is that there are cases where parties are still reporting donations late and not in the period to which they apply.
"And really we don't think that's good enough five years into the regime.
"In this case it's particularly the Conservative Party but other parties are not immune from it."