TONY BLAIR'S expected announcement today of a General Election on May 5 comes the day after a judge warned that the risk of postal voting fraud made Britain look like a "banana republic".

The Prime Minister is expected to go to Buckingham Palace to ask the Queen to dissolve Parliament on Thursday, with campaigning to begin next Monday.

Nearly all the polls suggest Mr Blair will be returned to Downing Street for an historic third term, albeit with a much reduced majority from the current 161.

But the election could be undermined by growing concerns that a switch to postal voting on demand - rather than only where a voter is away - will cast doubt on the result in many marginal seats.

Yesterday, a judge found six Birmingham Labour councillors guilty of an electoral fraud, at last year's council elections, which he said "would disgrace a banana republic", and ordered reruns of the polls in two wards.

Richard Mawrey QC, sitting as an election commissioner, told the vote-rigging trial that the postal voting system was wide open to fraud and accused the Government of being "in denial" about the danger.

The judge had heard testimony that activists made fraudulent applications for postal ballots and diverted them from the owners' addresses to "safe houses" for completion.

Meanwhile, witnesses alleged children and Labour Party supporters toured wards, taking newly-delivered ballot forms from letter boxes and intimidating householders into handing over votes.

Concern centres on the inability of election officers to check the validity of signatures on applications for postal ballots, because an entire household registers together.

The Government has dismissed warnings from the Electoral Commission, among others, that the system is unsafe without individual registration as just "scaremongering".

But Mr Mawrey said: "Anybody who has sat through the case I have just tried, and listened to evidence of electoral fraud that would disgrace a banana republic, would find this statement surprising."

The judge questioned whether those responsible for framing the postal voting guidelines could have made the fraud any easier to pull off.

He observed: "Postal ballots are sent out by ordinary mail in clearly identifiable envelopes.

"Short of writing 'Steal me' on the envelopes, it is hard to see what more could be done to ensure their coming into the wrong hands."

Last night, election officials at Sedgefield Borough Council - Tony Blair's constituency - agreed that applications for postal votes had to be "taken on trust" because signatures could not be confirmed.

The number of Sedgefield postal voters has soared from just 2,500 at the 2001 election to 8,300 for May 5 - and people can continue to register until six days before the poll.

A council spokesman said: "We process the applications as carefully as we can but, in the vast majority of cases, we are unable to verify the signature on the ballot paper.

"They are taken on trust, at face value. If every elector had to complete their own form to be on the electoral register, we could check their signature against their application for a postal vote."

Other North-East authorities have revealed huge rises in postal votes, including Gateshead (up from 5,600 in 2001 to 41,000), Hartlepool (1,884 to 7,000) and Newcastle (29,413 to 76,618).

Critics have predicted a repeat of Florida's "hanging chad crisis" at America's 2000 presidential election, with a spate of challenges to close results.

But Gateshead spokesman Jonathan Rew insisted postal voting was "convenient and popular", adding: "There have been no allegations of fraud in Gateshead."

If Parliament is dissolved on Thursday, the Government will have only two days to rescue as many as possible of 15 Bills that have yet to receive Royal Assent, in a period known as the "wash-up".

Legislation to introduce identity cards and outlaw incitement to religious hatred are expected to be sacrificed - allowing Labour to make them election issues.