Labour and the Tories have said all undeclared loans in last year's general election campaign complied with the rules covering party funding.
Labour and the Conservatives accounted for 84% of spending
The Electoral Commission had set a Wednesday deadline to confirm loans were made at commercial rates. If not, loans must be classed as donations.
Meanwhile, the police inquiry into Labour's loans from millionaire donors could be widened to look at corruption.
In all, political parties and campaign groups spent £42.3m on the election.
This is more than 50% more than was spent on the 2001 general election.
The Electoral Commission found Labour and the Conservatives each used up about £17.9m, while the Lib Dems spent £4.3m. The limit was £18m per party.
The UK figure for all the parties which put up candidates was £42,325,730, up from £26,689,901 in 2001.
It comes amid controversy about secret loans to Labour and the Conservatives.
The Electoral Commission had asked both parties to confirm, by Wednesday's deadline, that any undeclared loans were made at commercial rates - the only way they can avoid being declared in the same way as donations.
The Metropolitan Police announced last week it was investigating four complaints about Labour's loans from wealthy businessmen, under an act preventing the sale of honours.
It is now thought the inquiry could be widened to consider charges of corruption.
And a Scottish Nationalist MP, who made one of the original complaints, says the police have not ruled out investigating the Conservatives.
But a Scotland Yard spokeswoman said the complaints it was currently investigating all related to the Labour Party.
A Tory spokesman said: "We have reviewed our files with the benefit of legal advice and they fully comply with the Electoral Commission's rules.
"David Cameron has set out clear and detailed proposals to reform the funding of political parties in the future and the Conservative Party has already agreed to voluntarily disclose the source of any future loan agreements that the party enters into."
Labour are continuing to press the Tories to reveal the identity of their former lenders.
PARTY SPENDING PER VOTE
Conservatives - £2.03
Labour - £1.88
Lib Dems - 72 pence
The Tories have refused, saying it would breach their backers' confidentiality.
The largest proportion of money spent on the 2005 campaign - 37% - went on advertising, followed by 22% sending material to electors and 10% on rallies and events.
Electoral Commission chairman Sam Younger said: "We are pleased to report real progress in the understanding of the law and in the quality of returns from the parties - without financial transparency, the public's perception of the integrity of the democratic process is damaged.
"However, we are aware that further work is needed to make the sources of political funding clearer, whether it is money for election campaigns or the day to day running of parties, particularly on the issue of loans."
The commission's figures mean the Conservatives spent £2.03 for each of its 8.79 million votes at the last general election.
SPENDING ACROSS UK
England - £34.9m
Scotland - £3.6m
Wales - £2.2m
Northern Ireland - £556,552
Source: Electoral Commission
For Labour - with 9.57 million votes - it was £1.88 and the for the Lib Dems - with 5.99 million - it was 72p.
The fourth-biggest election-spending party was George Galloway's Respect, on £320,716.
This was followed by the Ulster Unionist Party, with 251,119, the Scottish National Party, with 193,987, and the SDLP, with £154,088.
The lowest amount spent was £18, by the Personality AND Rational Thinking? Yes! Party, which only had one candidate.
The commission heard from 3,504 of the 3,554 candidates in 646 constituencies across the UK.