Mr Howard listed a catalogue of electoral fraud claims at PMQs

Tory Leader Michael Howard told MPs his party chairman had already asked the Electoral Commission to oversee the Birmingham and Leicester by-elections.

The prime minister promised to consult with colleagues and investigate the implications of the proposal.

Mr Howard listed fraud claims linked to the recent elections in Birmingham.

The "serious allegations" included claims voters were put under pressure to vote a certain way, that a councillor was allegedly seen sorting through ballot papers in a car park and that a postman was offered �500 for his sack of postal votes.

The allegations are contained in a petition to the High Court written by the leader of the Liberal Democrat group in the city, Councillor John Hemming.

It is important for all political parties to stamp out corruption or malpractice wherever it exists Tony Blair

He and two colleagues are calling for the results in Aston ward to be rendered null and void in the light of the allegations.

While the People's Justice Party has questioned the results in nearby Bordesley Green ward - part of the Birmingham Hodgehill constituency which is going to the polls for a Westminster by-election in mid-July.

'Urgent action'

Mr Howard said alleged incidents such as these "call into question the integrity of our electoral system" and urged "all mainstream parties to work together to stamp them out".

Mr Blair responded saying: "of course, I agree it is important for all political parties to stamp out corruption or malpractice wherever it exists".

Tory party co-chairman Liam Fox wrote to the voting watchdog on Tuesday saying "urgent action" was needed to be taken to safeguard the electoral system.

"In view of the forthcoming by-elections in Leicester South and Birmingham Hodgehill I am writing to you to ask the Returning Officers for both seats permission to install Electoral Commission observers at polling stations, and at the opening of postal ballots," Dr Fox's letter said.

Impractical

Such a move would "promote the transparency of the democratic process", he argued quoting one of the watchdog's own reports.

Coun Hemming welcomed the Conservative proposal but told News Online he thought it was impractical.

"The fact is the Electoral Commission would have to send about 300 people to monitor the streets for about a fortnight in order to monitor the election properly," he said.

An Electoral Commission spokeswoman confirmed it had received Dr Fox's letter.

"We have a duty to report on general elections, European parliamentary elections and pilot schemes, but we have no remit to report on parliamentary by-elections," she added.