The Conservatives failed to win an overall majority at the general election because of electoral fraud, the party chairman has said.
Baroness Warsi told the New Statesman the alleged fraud was "predominantly within the Asian community" and benefited Labour.
Labour called the allegations "unsubstantiated" and urged Lady Warsi to produce evidence.
The Electoral Commission said police would need to investigate any claims.
The Conservative Party chairman told the New Statesman: "At least three seats where we lost, where we didn't gain the seat, based on electoral fraud. Now, could we have planned for that in the campaign? Absolutely not."
She would not say where it had happened: "I think it would be wrong to start identifying them.
"It is predominantly within the Asian community. I have to look back and say we didn't do well in those communities, but was there something over and above that we could have done? Well, actually not, if there is going to be voter fraud."
Lady Warsi said she had written to Deputy Prime Minister Nick Clegg, who is overseeing the coalition government's electoral reforms.
A Conservative spokesman confirmed the party did have some concerns about a number of seats which it was looking into.
The BBC tried to contact Lady Warsi about the allegations but she was unavailable.
The Conservatives won 307 seats and 36.1% of the votes, while Labour won 258 seats and 29% of the votes.
The Liberal Democrats won 57 seats with a 23% share of the votes.
The Electoral Commission, which is publishing its own report on the matter in January, told the BBC without any further details about the constituencies involved, it would be unable to comment further.
The city of Birmingham was once described by High Court judge Richard Mawrey QC as "worse than a banana republic" because of the scale of postal ballot fraud when it came to city council elections.
Before this election, Birmingham City Council and the police said new security measures would make postal voting in the general election "safe and easy".
Although there has been no suggestion that Birmingham is among the areas Lady Warsi is referring to, Khalid Mahmood, the Labour MP for Birmingham Perry Barr, told the BBC that Lady Warsi should share any evidence she had obtained with the authorities.
"She needs to substantiate that. She needs to give the evidence to the authorities and make sure it's investigated.
"And if she has the evidence [then] to deal with it. I can't see that happening."
He said he would be amazed if there was evidence of people conducting wide-spread electoral fraud during the general election.
John Hemming, the Liberal Democrat MP for the Birmingham Yardley constituency, said more needed to be done to safeguard against voter fraud.
"In my own constituency I've had ghost voters who don't actually live where they claim to live.
"But the underlying problem with the system is that once somebody has turned up to the polling station and pretended to be somebody else to vote, it's all gone."
He said it was difficult to get evidence that fraud was taking place.
A spokesman for Scotland Yard said police received 41 complaints about the election but was taking no further action in relation to those.