A police investigation was launched last night after postal votes from the Ealing Southall by-election were allegedly leaked.
The Daily Telegraph website published the results of postal ballots, showing a close-run race between the candidates.
This was swiftly pulled but Labour launched an official complaint, pointing out publication of the results before the close of polls breached electoral law.
Media organisations are prohibited from communicating results from postal ballots or exit polls until after counting has begun in case it influences voters.
Anyone found guilty of a leak can be sentenced to six months in prison.
The Daily Telegraph said it obtained the information from a source inside the Conservatives' campaign.
The Conservatives said their agent in Ealing Southall had not published the information or spoken to the newspaper.
A party spokesman said they had not seen the blog and accused their opponents of "mischief making" in raising their concerns.
Labour's election agent Ken Clarke has filed a written complaint and said the publication amounts to a "clear breach" of electoral law.
A Met spokesman said: "I can confirm the Metropolitan Police Service 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."
Party officials think the results were published in an attempt to galvanise the anti-Labour vote behind the Tories.
While Labour are on a clear course to retain their Sedgefield seat, the Ealing Southall by-election is a key test of all three major party leaders.
David Cameron has visited the constituency five times and is eager to show the new-look Conservatives can take seats from Labour and appeal to ethnic minorities.
Labour currently hold the seat and will be eager to retain it.
Gordon Brown is thought to be watching the by-election with interest, and a strong performance by Labour could encourage him to call an election in spring 2008.
The Liberal Democrats have to prove they are a viable electoral force.
Menzies Campbell yesterday made his seventh appearance in west London. He almost certainly faces questions over his leadership capabilities if the Lib Dems perform badly today.