Claims of election fraud are being investigated by police after fake election pamphlets were reportedly handed out in the name of a candidate.
Leaflets purporting to be from a Liberal Democrat candidate appeared to pledge support for a number of causes, including a controversial planning decision.
They were handed out the day before the Richmond Upon Thames Council by-election in south west London on May 4.
A Lib Dem spokesman said candidate Jane Dodds had not backed the manifesto points on the fake flyer.
The hotly contested Richmond North seat was won by Conservative candidate Stephen Speak, who beat Ms Dodds by 146 votes.
A Scotland Yard spokesman said officers were looking into the claims. He said: "The Metropolitan Police Service is aware of an allegation of a breach of election legislation in Richmond.
"We are currently making an assessment to ascertain whether there is evidence to support the complaint."
The by-election took place at the same time as the London Assembly and London mayoral elections.
Stephen Knight, leader of the Liberal Democrats at the council, said: "It was a ward which had been won by a Conservative candidate by 19 votes two years ago. It is a very marginal ward indeed.
"The night before polls opened, the leaflets were delivered between 10pm and midnight - under the cover of darkness. There were three different versions for the different polling districts in the ward."
He said the flyers used the logos and font of the original flyer for Ms Dodds, but the content had been changed.
The leaflets claimed Ms Dodds believed a controversial development in the ward should be used for housing homeless immigrants, or to become a high-rise block.
"They were clearly designed to defect support from our candidate by purporting to back things which were unpopular," Mr Knight added.
"The result was close, it may have gone the other way if this had not happened."