Labour leaders came under fire today after apparently admitting they had been given a sneak preview of secret postal ballot forms for the city election.

Ken Bodfish, Labour's leader on the city council, had warned party members the result was hanging on a knife-edge and urged candidates in "four key wards" to step up the pressure.

Members of all parties were invited to attend a preliminary check of received votes in advance of the official count when the poll closes on Thursday.

In a subsequent letter to party members, leaked to us today, Mr Bodfish said he had "firm evidence" of how the voting was going.

He wrote: "The first postal ballots are being opened and checked before the count next Thursday.

"During this council administrative procedure we are able to 'sample' how people are voting. The bad news is that we need to be doing better than we are.

"The good news is that it will only take a small number of extra Labour votes to secure a Labour council.

"The votes already cast back up what we have been saying for some time. Whether it will be a Tory council or a Labour council on May 2 depends on how we do in four key wards."

Coun Bodfish used the rest of the letter to ask members to do all they could to help Labour retain control of the council.

Green group convenor Keith Taylor said his party had always had deep reservations about the all-postal system and these had been heightened by Labour's claim it had been able to take an early sample.

He added: "This we believe to be contrary to the guidance on postal ballots and represents a gross abuse of the system.

"The leaflet was distributed to Labour Party members, a number of whom forwarded copies to us, as they themselves were concerned."

Labour election organiser Simon Burgess, who like Coun Taylor is a candidate, said the postal ballot was being supervised by the Electoral Reform Society in London.

He added: "All parties can go along to the opening of postal ballots. I went there and the Tories also sent a representative. The Greens did not."

Mr Burgess added: "I would like to make it clear that we have no idea of how any individual voted."

He declined to say which were the four key wards. He said the letter had been sent out in a bid to galvanise party workers in what was likely to be a close contest.

The Electoral Reform Society said it was unable to comment as the ballot is being supervised by its subsidiary, Electoral Reform Services.

A city council spokesman said: "We have followed very strict procedures. Checks have been made to see that votes have been cast legally but not on the way people have voted."