Allegations of dirty tricks overshadowed the final day of campaigning for today’s by-election in Ealing Southall as the Tories accused Labour of orchestrating an ambush on David Cameron.

The Conservatives claimed to have identified a woman who confronted Mr Cameron during a walkabout in the constituency as a Labour supporter involved in the party’s campaign. The woman, who appeared to be a passer-by, was involved in a heated exchange with the Tory leader as she criticised proposals from one of his policy groups for tax breaks for married couples. The exchange, last Friday, was widely reported, including in The Times.

The Tories said yesterday that the woman looked remarkably similar to a Labour supporter wearing a party sticker whose photograph appears in a by-election leaflet with Virendra Sharma, the party’s candidate.

In a separate development last night, Scotland Yard confirmed it was investigating an allegation of “possible electoral offences” that would be “thoroughly investigated”.

The complaint related to claims by a Labour official that the Tories had broken electoral law by disclosing postal vote results.

Under the Representation of the People Act, which governs elections, it is illegal to publish statements relating to the way people have voted before the close of polls.

Labour pointed to a blog called Little and Large by Carlin and Isaby, which claims to have been given figures from the postal vote tally by a “source” in the Tory campaign.

Labour’s election agent Ken Clark told police in a letter that this appeared to be a “clear breach” of electoral law and asking for the matter to be investigated “as a matter of urgency”. The posting was removed from the internet last night.

Referring to Mr Cameron’s heated exchange, Grant Schapps, the Tory MP running his party’s campaign, said: “The Labour Party is resorting to dirty tricks by a party apparatchik posing as a member of the public to harangue David Cameron.” In a potential embarrassment for Labour, the leaflet features a prominent photograph of Gordon Brown, who has promised an end to spin and a new style of politics that reconnects politicians with voters.

The confrontation between the woman and Mr Cameron was one of several difficult encounters for him during a 30-minute walkabout in West Ealing. Immediately after their clash, The Times asked her for further information about her views. She said that she would give it but wanted to telephone her mother first and walked away, apparently conducting a telephone conversation.

Douglas Trainer, a member of the Labour campaign team who attended Mr Cameron’s walkabout, denied knowledge of any orchestrated confrontations with Labour supporters. Mr Trainer, a former lobbyist and special adviser to Jack McConnell, previously Labour’s First Minister in Scotland, said that he was present to feed back to the Labour campaign about Mr Cameron’s walkabout and monitor his comments about deselected former Labour councillors who joined the Tories and whom the Tories incorrectly described as defectors.

Labour issued a carefully worded response when asked by The Times if the party knew who the woman was and whether she had confronted Mr Cameron at the behest of the Labour campaign. Tom Watson, the Labour whip helping to run the campaign, said: “I have no idea who this woman is, but I can say that there is no Labour Party dirty tricks campaign. However, I am not at all surprised if a Labour Party supporter might wish to confront David Cameron on one of his many walkabouts in Ealing Southall.”

The claims and counter claims marked the culmination of the most bruising by-election campaign for many years, which was soured by controversy over disputed Tory claims of defections from Labour and a ferocious Labour assault on the Tory candidate. Mr Schapps described as “abusive and potentially libellous” a Labour leaflet describing Tony Lit, the Conservative candidate, as a “Tory playboy”, pointing out that he was married with three children.

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