Disillusionment with Tony Blair and Labour is so entrenched that many voters will assume the government is guilty in the cash for honours affair even if no charge is ever brought, opinion pollsters said on Friday.

Experts also said the longer the police inquiry into the allegations drags on, the greater the risk that voters’ loss of trust in Mr Blair contaminates the Labour brand as a whole, potentially damaging the electoral chances of Gordon Brown, his expected successor.

The polling evidence helps to explain why senior Labour figures reacted with such alarm to the damaging headlines that on Friday trumpeted the news that Mr Blair had been interviewed again by the police. Lord Kinnock, the former Labour leader, warned that “the damage to reputation, to trust . . . will take years and a great deal of action to try and repair”.

Ministerial pleas to the public to await the end of the investigation seem likely to fall on deaf ears. David Miliband, the environment secretary, argued on the BBC that “it’s really important that we maintain a British presumption that you are innocent until proven guilty”. But polls suggest many voters have already judged the prime minister’s integrity and found him wanting.

When New Labour swept to power in 1997, Mr Blair’s public standing was so strong that he was able to deal easily with allegations over a £1m donation from Bernie Ecclestone, the Formula 1 racing magnate. But the prime ministerial assertion then – “people think I’m a pretty straight kind of guy” – is unthinkable today.

More than 55 per cent of people now characterise the government as “very sleazy and disreputable”, according to a YouGov poll last month. In the same survey, 64 per cent said they believe Labour “probably” did offer honours to wealthy businessmen who promised large loans or donations.

A failure to prosecute anybody over the allegations may not affect this damaging verdict. Andrew Cooper of Populus, the polling organisation, said: “If no charges are brought . . . lots of people will assume it’s been rigged. They will be cynical about it because they’re cynical about every aspect of politics.

“The details of this sort of story tend to wash over a lot of people. The great majority already made up their mind about Blair and most are heartily sick of him.”

A crucial concern for Labour is the extent to which the Blair effect could taint his successor by association. A Mori poll yesterday found roughly equal numbers of voters – 68 and 69 per cent – expressing dissatisfaction with the prime minister and the government.

Anthony King, professor of politics at Essex University, said this loss of trust in politicians was not focused on the prime minister. “It applies to Labour and to politicians in general. The belief is near universal that they all do it.”

The chancellor’s ability to mitigate the political damage from this public cynicism, once he moves into Number 10 this summer, depends in part on the allegations, which are denied by all concerned, not resulting in any charges. A prosecution could put Mr Blair in the witness box, seriously undermining Mr Brown’s efforts to regain voters’ trust. John Curtice, professor of politics at Strathclyde University, said: “If charges are brought, and there is a case, Tony Blair will go and the problem will still be there for the Labour party.”

Assuming there is no prosecution, however, much of the electoral sting of the affair could vanish with Mr Blair’s departure. Mr Cooper argued that the public perception of Mr Brown as a “dull, Scottish bank manager,” in contrast to Mr Blair’s salesman image, would make it easier for the chancellor to draw a line under it.

Mr Brown will also draw comfort from David Cameron’s seeming inability to make much political capital from the government’s woes. The leader of the Conservatives has taken his party to a three- to four-point lead over Labour in polls over the last year, but has yet consistently to break though the 40 per cent rating needed if he is to be confident of gaining power at the next election.

Mike Smithson of the politicalbetting website said: “Extraordinarily, given everything over the past few months, the polls have been reasonably good for Labour.

“It’s easy to overstate the effect [on voters] of issues like cash for honours,” he added.

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