Today's hotly contested by-election in Ealing Southall will take place amid a police investigation into electoral fraud allegations and claims by some residents that they have been robbed of their chance to vote.

Yesterday the Metropolitan Police launched an inquiry after a complaint from Labour that an apparent reference to results of the postal votes was published on the Internet.

It is illegal to publish such information before polling has closed.

Yet a blog on the Daily Telegraph's website quoted "a source inside the Tory campaign" as saying the Ealing Southall result looked close.

The source based his prediction on party activists who saw ballot papers as they were verified yesterday, ahead of them being counted today.

Labour's election agent, Ken Clark, wrote in a letter to police that it appeared to be a "clear breach" of electoral law.

The Met said in a statement: "We can confirm the Metropolitan Police has today received an allegation of possible electoral offences in relation to the Southall by-election.

"The allegation will be thoroughly investigated and appropriate action taken."

The blog has since been removed from the website.

"A blog was filed at about 6pm," a Telegraph Media Group spokesman said.

"It was on the site for a short period of time until it was pointed out that it might be in breach of electoral law and it was removed instantly this was pointed out."

Meanwhile some Ealing residents have complained that they have been robbed of their chance to vote in the election, which is seen as a big test for the major political leaders.

Andrew Burnett told the Ealing Times, sister paper of Local London: "I received my polling card on July 9, five days after the closing date for postal votes and 48 hours before the closing date for proxy votes.

"I am not able to vote in person on July 19, and have not been able to find a proxy voter in the very short time available, and so I feel that I have been deprived of my vote."

A spokesman for Ealing Council said: "The timescale for the by-election was set by the Electoral Commission.

"Unfortunately the timing was very tight and there was very little time."

He added that "we have not had any major complaints about this issue".

Andrew Gough, from Hanwell, told the Ealing Times he had not received a polling card at all, and asked why the council has not given details of where people can vote.

The Ealing Council website does not have details of polling booth locations.

Prime Minister Gordon Brown, Leader of the Opposition David Cameron, and Sir Ming Campbell, leader of the Liberal Democrats, as well as host of other national political figures, have swept through Ealing Southall in the last few weeks in an attempt to sway the voters.

The by-election was triggered by the death of veteran MP Piara Khabra, and is being seen as a test for all three leaders as they fight for political momentum.

The fight to take over from veteran MP Piara Khabra in Ealing Southall has been hotly contested between 12 candidates: The Conservative candidate, Tony Lit, the wealthy son of Sunrise radio owner, Avtar Lit. The married father-of-three is involved with regeneration projects across west London as a director of not-for-profit Heathrow City Partnership Ltd.

Sarah Edwards is the only woman in the by-election, and is representing the Green Party. She is campaigning for the seat on the issues of climate change, public transport and against the war in Iraq.

Cllr Virendra Sharma is the Labour candidate. Mr Sharma represents the Norwood Green area of Southall on Ealing Council, and has been serving the borough for 25 years. The married father-of-two and grandfather-of-three is campaigning for safer streets more affordable housing and better public transport.

The Liberal Democrats' campaign has been led by Hanwell resident Nigel Bakhai, who is standing again after coming second in the last General Election.

The equipment demand planner is campaigning on the NHS, crime and the war in Iraq.

The Official Monster Raving Loony Party candidate is also its shadow minister for chocolate, John Cartwright. He wants to provide voters true progressive politics.

Teacher and local campaigner Salvinder Dhillon is the Respect candidate. He is campaigning against the war in Iraq and privatisation and is in favour of better public services. The married father-of-three grew up in Southall and has locally campaigned to keep Southall Community Centre and the Dominion Centre open.

The UK Independence Party candidate is Dr Kunnathur Rajan. Dr Rajan has worked in the NHS for 28 years and moved to the country in 1960. Indian-born Dr Rajan wants an end to uncontrolled mass immigration, a referendum on the EU treaty, and a better NHS for the country which he says has collapsed under the Labour government.

Other candidates are Nairobi-born businessman Sati Chaggar for the English Democrats, who wants justice and fairness for all people whatever their ethnicity; Yaqub Masih is representing the Christian Party and works as general secretary to UK Asian Christian Fellowship; Dr Jasdev Singh Rai, Kuldeep Singh Grewal, and Gulbash Singh are standing as independent candidates, although Mr Grewal is backing the Labour candidate, and Gulbash Singh is said to have backed Tony Lit.

A result is expected in the early hours of Friday morning.