David Cameron has sanctioned a return of the Tories as the nasty party, smearing Labour over its links with the Co-operative movement, Yvette Cooper, the shadow home secretary has claimed as she denied the party was in financial trouble due to the possible withdrawal of Co-op Bank funding.

The former chancellor Alistair Darling also expressed fears about the independence of the inquiry announced by the chancellor, George Osborne, into how the Co-op Bank created £1.5bn hole in its balance sheet.

He said he feared the inquiry will take for ever and a day.

Cooper insisted she had been shocked at the revelations about the Rev Paul Flowers, the former chairman of the Co-op Bank, who has emerged to be a drug user and fan of gay porn.

Michael Gove, the education secretary, said the Conservatives did not want a dirty election campaign but claimed: "Whenever anyone asks questions about the Labour party, Ed Miliband has a sort of coquettish reticence," referring to Miliband's refusal to answer questions on Labour's approach to the Co-op and allegations of vote-rigging by the Unite union in Falkirk. Cooper's husband, the shadow chancellor, Ed Balls, received £50,000 from the Co-op Group.

Cooper said: "It is a return to the nasty party, I don't think John Major would ever have done this, I don't think Margaret Thatcher would have done this. They've a different style of politics, this is David Cameron's approach," she told BBC1's Andrew Marr Show.

Her remarks came after Miliband claimed that the prime inister's recent attack on Labour over the affair "demeans his office".

In an article for the Independent on Sunday, the Labour leader says: "His main political strategy is now to sling as much mud as possible in the hope that some of it sticks."

But Gove also praised Miliband and his election co-ordinator, Douglas Alexander, for having told Gordon Brown to get rid of Damian McBride, his chief spin doctor, who has subsequently admitted he briefed personalised stories to undermine Brown's opponents. Describing McBride as a hit man, Gove said: "To his credit Ed Miliband and Douglas Alexander said to Gordon Brown 'get rid of this chap' and they did not."

He added: "I think Ed Miliband wants to fight a clean election campaign. I certainly do and I know that David Cameron does … the election campaign should be conducted on the basis of policy versus policy".

Gove said one of his worries was that a lot of the Labour critique of the government had been based on the background of politicians, such as where they went to school.

He said the government was willing to discuss its own role in regulating the Co-op Bank. He said: "Chancellor George Osborne took the advice to set up an inquiry to ask searching questions about what went on right up to the moment when we discovered what we have discovered about Paul Flowers." .

Gove said Labour's difficulty was they were "the people responsible for allowing Paul Flowers to be appointed on their watch. There is a pattern of behaviour here so that when tough questions gets asked about Flowers, Unite, Grangemouth, ballot-rigging, they tend to clam up". Miliband, he said, was a great advocate of judge-led inquiries but "whenever someone asks questions about the Labour party there is a coquettish reticence".

Darling, who was chancellor at the time of the Co-op's ill-fated merger with the Britannia Building Society, said he would be welcome an inquiry, but added it had to be truly independent and not take "for ever and a day". Some newspaper reports have suggested an investigation would take two years.

He said the regulators needed to explain why it was satisfied to appoint Flowers the Co-op rank chairman since he was "manifestly unsuitable to be chairman of any PLC or mutual bank".

He said the inquiry needed "to look at the facts of the case, rather than politics that seems to be driving things".

Grants Shapps, the Conservative chairman was forced on the back foot on BBC Sunday Politics when he said it would be for an independent inquiry to decide why the Conservative-run Treasury in the summer of 2012 was still promoting the Co-op as the preferred bidder for 600 Lloyds branches. It was argued the Treasury should by that stage have seen the Co-op had hit trouble, and could not take over the Lloyds branches. The bid collapsed largely after Lloyds recognised there was a hole in the Co-op Bank balance.

Shapps also shifted ground saying he was not alleging that Miliband personally knew about Flowers' background saying instead it was legitimate to ask why the Labour leader did not know about Flowers' background. He said: "The question is if Ed Miliband didn't know about that, why didn't he know about that?"

Martin Wheatley, the chairman of the Financial Conduct Authority defended the work of its predecessor the Financial Services Authority, saying the FSA was constantly pushing the Co-op asking whether "you have not got the people, the capital and the system in place and they did not".

He also said the FSA challenged the appointment of Flowers as chairman even though it did not have the powers to do so, or even require to be him interviewed. The FSA formally only had powers to challenge an appointment to a bank board.

He said the FSA told Flowers that he did not have the right experience to be bank chairman and that he needed to add bank experience onto the board.