Political parties were urged to improve their disclosure of financial support yesterday after more than £300,000 of donations were declared late.
The Electoral Commission said late reporting was "not acceptable" as it emerged the Tories received almost £217,000 in 2005 without declaring the money on the Register of Donations.
Labour, the Greens and the UK Independence Party also made disclosures which were at least six months overdue and there were more to be declared.
The late declarations came with the quarterly update of the Register of Donations, which included loans for the first time since the cash-for-honours row.
The Labour Party reported more than £28 million, the majority of which was received before the latest accounting period of April to June.
The Tories declared more than £2.8 million of loans the Electoral Commission said had been received in the second quarter of 2006.
The Liberal Democrats' declaration of £584,000 is understood to include donations from prior to the reporting period.
The Electoral Commission said it was "disappointed" not all parties had declared loans voluntarily ahead of September 11, when statutorily obliged to.
Chief executive Peter Wardle said: "Full disclosure of donations and loans is vital to address public concern about the transparency of party funding.
"The fact the main parties seem to be struggling with disclosure we have called for on a voluntary basis suggests they are ill-prepared to comply with the new laws.
"We will do all we can to ensure parties understand the requirements."
Figures for the second quarter of 2006 showed £10.7 million had been donated to 16 parties.
Labour received £3.4 million, the Tories £5.9 million and Lib Dems £783,000.
Another £309,639 was declared for 2005 which should have been registered by the end of February.
They included £216,933 to Conservative Party associations, £9,500 to Labour, £27,543 to the UK Independence Party and £1,200 to the Greens.
These donations only came to light when spotted in parties' annual accounts. The Electoral Commission said there were more.
"Parties have told the Commission there are further donations which should have been reported, but which do not appear in today's figure," it said. "The Commission has given the parties until September 30, 2006 to report these."
The Commission added: "Whilst the vast majority of donations are reported on time, the Commission believes that late reporting of donations is not acceptable.
"We will continue to highlight the instances where donations are reported late and we expect the parties to improve their reporting systems."
The three main parties reported loans totalling more than £31.5 million.
Labour declared £28,200,693.85, the Tories £2,812,000 and Lib Dems £584,239.08.
"The Commission is disappointed that, despite public commitments, not all parties have made sufficient efforts to provide details of loans to their accounting units, and in some cases to national headquarters," it added.
"Given this, it is clear parties will need to make immediate efforts if they are to be in a position to comply with the requirement in the Electoral Administration Act to report loans on the same basis as donations from September."
There was confusion earlier when the Electoral Commission's literature appeared to show that Labour's newlydeclared loans had been received in a three-month period this year.
The Commission clarified later that the loans had been declared, rather than received, during the reporting period April 1 to June 30.
The parties had interpreted the voluntary code in different ways with the Tories reporting only those received during the period.
Labour and the Lib Dems had declared previous loans as well, the Commission said.