The power of the prime minister's two most senior advisers should be curbed, a committee of MPs said on Tuesday.

The public administration committee concluded that Alastair Campbell and Jonathan Powell had unprecedented rights and powers and has called for an examination of the growing number of ministerial fixers.

The MPs are calling for an inquiry into the role, function and powers of special advisers and believe the system must be made more "transparent".

Tony Blair and Powell were criticised in the report for refusing to give evidence before the committee.

"This has made our work more difficult and less complete than it should have been. The fact that the committee's invitation to both the prime minister and chief of staff have been declined make it especially difficult for us to form a view about the role of special advisers at the centre of government," said the MPs.

It also found that the number of advisers in government departments had risen to 78 - at a cost of £4.4 million-a-year.

Alastair Campbell, the prime minister's press secretary, has already been criticised for his attacks on the Conservative Party rather than focusing on representing the prime minister's views. Sir Richard Wilson, the cabinet secretary, had been forced to re-issue Campbell with a reminder of the rules governing special advisers.

The report called for renewed vigilance as the election nears. "Tensions in this area are inevitable, which is why continuing vigilance is appropriate in policing the boundary line. In particular, advisers are prohibited from taking part in the work of their party's national organisation; and although they may continue, during elections, to give specialist or political advice to their ministers, they must be careful not to take any part in the campaign going beyond the provision of such advice,'' the report said.

Conservative committee member Andrew Tyrie dismissed his colleagues conclusions as a "whitewash".

He claimed the Labour dominated committee had exonerated Campbell and Powell because of party bias. "Special advisers, if they become too numerous, can prejudice the neutrality of the civil service. What we've got now is a massive campaign team for the re-election of the Labour Party, paid for by the taxpayer," said Tyrie.

Labour has hit back at Tory criticism by attacking William Hague over the issue of Short Money spending.

Seizing on a memorandum that says "the auditor for the Official Opposition's Short money expressed difficulty in certifying that Short Money had been spent on parliamentary business", local government minister, Hilary Armstrong, said on Tuesday: "William Hague must publish full details of what he has spent the Tories Short money on to prove he has not misspent the money - or hand back the missing millions straight away."

"When Labour was in opposition Short Money was spent on parliamentary research. The Tories now abuse their £3 million a year to pay for their spin doctors and war room," she said.