An investigation has been launched after a complaint from the Labour Party about alleged electoral offences in the Ealing Southall by-election.
Polls open in the Ealing Southall constituency on Thursday
It relates to a Daily Telegraph website blog entry which claimed to show how many postal votes each candidate had won ahead of Thursday's ballot.
The Telegraph Media Group said this was removed as soon as it was pointed out that it could breach electoral law.
The Metropolitan Police said the matter is being "thoroughly investigated".
The article on the Daily Telegraph website quoted "a source inside the Tory campaign".
However, a spokesman for the Conservative Party said: "Our agent in Ealing Southall hasn't published any such information or spoken to the Daily Telegraph."
A Telegraph Media Group spokesman confirmed the blog was filed at 1800 BST.
He added: "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."
The complaint was made in writing by Labour's election agent Ken Clarke, who told police the publication of purported postal voting results appeared to be 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."