The following correction was printed in the Guardian's Corrections and clarifications column, Friday 15 May 2009.

In the article below, we said that Hazel Blears, the communities secretary, avoided paying about £18,000 in capital gains tax on the £45,000 profit she made from selling a flat in Kennington, south London, in 2004. This would have been a maximum, but our calculation failed to allow for the possibility of taper relief and any unused part of her annual capital-gains exempt sum. Indeed, the next day Blears announced that she was sending a cheque to the tax authorities for £13,332 as payment.

David Cameron finds the ­Conservative party in the firing line over the highly charged issue of MPs' expenses and allowances, as embarrassing details about the claims of a series of shadow ­cabinet ministers are leaked.

As a former deputy speaker of the ­Commons warned a general election may have to be held to resolve the expenses crisis, the Tory leader moved last night to seize the political initiative by apologising for the way his MPs had attempted to ­maximise their benefits.

"Tomorrow is going to be another bad day for parliament and, frankly, a bad day for the Conservative party," Cameron said. "We have to acknowledge just how bad this is. The public are really angry and we have to start by saying: 'Look, this system that we have, that we used, that we operated, that we took part in – it was wrong and we are sorry about that'."

Cameron's intervention came as the Daily Telegraph, which has faced criticism for focusing on Labour MPs in the three days since it started publishing leaked details about parliamentary expenses, shifted its attention to the shadow cabinet. Some are accused of adopting the practice of "flipping" their homes – the process in which MPs changed the designation of a house as their second property – allowing them to claim expenses. The shadow ministers under pressure are:

• Michael Gove, the shadow schools secretary and a member of Cameron's inner circle, spent more than £7,000 furnishing a London property in 2006 before "flipping" the second home designation to a new one in his Surrey Heath constituency.

• Andrew Lansley, the shadow health secretary, spent thousands of pounds renovating a thatched Tudor country cottage before selling it. He then moved the second designation to a London flat.

• Alan Duncan, the shadow leader of the Commons who chairs the Commons audit committee which oversees MPs' expenses, had a claim for £3,194 ­gardening expenses declined in March 2007. He says this hapened after he raised the matter with the Commons authorities.

• Francis Maude, the shadow cabinet office minister who is leading the Tories' preparations for government, tried to claim mortgage interest on his family home in Sussex. This was declined by the Commons fees office.

• Chris Grayling, the shadow home ­secretary and another member of the Cameron circle, claimed for the ­renovation of a London flat which is 17 miles from his family home.

• Cheryl Gillan, the shadow Welsh ­secretary, claimed for dog food. She has agreed to repay the claim.

• Oliver Letwin, who is in charge of the Tories' general election manifesto, charged £2,000 to replace a leaking pipe under a tennis court. The pipe was not related to the court and Letwin was obliged to mend the pipe after an order from the local water authority.

• David Willetts, the shadow ­universities secretary, claimed more than £100 for workmen to replace 25 lightbulbs at his home.

All the shadow ministers issued detailed statements declaring that they had acted within the rules or, in the case of Gillan, apologising and offering to refund the money. One Tory source said that no rules had been broken, but added: "We have a perception problem."

The series of statements – and Cameron's apology – showed the Tories were well-prepared. They were told on Friday the focus would turn to them today, though they were not informed about the individual claims until 11am yesterday. Labour was not told about the Telegraph series until 3pm on Thursday, hours before the publication deadline.

The focus on the Tories is likely to turn the expenses leaks into a wider crisis for the political classes. The impact was highlighted by Lord Naseby, a former Commons deputy speaker, who said the leaks were so damaging that parliament might have to be dissolved. "It's dreadful. It is quite awful," the Tory peer told BBC Radio 4's The World This Weekend. "I think, frankly, if this runs and runs, then parliament should be dissolved, I think they have to start again. The Great British public has lost their confidence and I think that it is extremely serious. And if it is that serious then there is only one way of dealing with it, that is to dissolve parliament."

His remarks came as Hazel Blears, the communities secretary, faced a fight for her political life after apparently making contradictory declarations about a publicly subsidised flat to avoid a £18,000 tax bill.

Blears appeared to have avoided paying capital gains tax when she sold a flat in Kennington, south London, in August 2004 for £200,000, making a profit of £45,000. To avoid paying tax of about £18,000 on the profit, she would have had to declare the flat to the Inland Revenue as her main residence. But in April 2004 she designated the Kennington flat as her second home to the Commons ­authorities. This allowed her to claim mortgage interest payments on it of £850 a month. Blears said yesterday she had done nothing wrong.

MPs will make clear the need for change when they agree to a tougher system to audit their expenses. Sir Stuart Bell, the veteran Labour member of the Commons Commission, said: "In all probability the commission will approve a special specific audit unit, hived off from the fees office, independent of the fees office, which will verify in future every claim that's made by any MP."

Under the new system, officials who approve MPs' claims will be overseen by senior officials who will be formally charged with advising on whether claims are valid. A separate audit office in the Commons will conduct statistical analysis to identify MPs who overclaim. Since 1 April this year the National Audit Office has been auditing MPs after they were placed on the same level as the rest of the public sector.

The internal and external auditing marks a major change. Until now MPs' expenses have only been examined individually, with no overall oversight. By conducting a statistical analysis, the auditors are meant to identify MPs who claim above average amounts.