Image copyright Getty Images Image caption Researchers say voters born overseas make up 10% of the electorate

Migrant voters could have a "decisive" impact in key marginal seats at the general election, a report suggests.

Nearly four million people eligible to vote were born overseas, according to the study by Manchester University and the Migrants' Rights Network.

In 70 constituencies, migrant voters outnumbered the majority of the sitting MP, it said.

And negative messages about immigration could alienate these people before the vote on 7 May, the authors warned.

The research, based on data from the 2001 and 2011 censuses, estimated a record number of migrant voters - nearly 10% of the electorate - were eligible to take part.

Migrants could make up more than a third of the voters in about 25 seats in England and Wales and at least a quarter of the electorate in more than 50 seats, the report said.

In two constituencies - East Ham and Brent North - more than 50% of potential voters were born overseas.

'Heated debate'

"In a small number of marginal seats, the migrant vote really could be decisive," co-author Ruth Grove-White told BBC Radio 4's Today programme.

Acknowledging that migrant voters were a "highly disparate" group, she warned there were "certain issues which unite migrant voters - and one of them is their view about immigration".

"We know that migrants in the UK are much more positive about immigration and its cultural and economic impact, and much more likely to be concerned about racial discrimination," she added.

"Politicians need to be careful. This is an election which has been characterised by a very heated debate about immigration. It's very easy to forget that politicians are also talking to migrant voters as well."

Image copyright Getty Images Image caption Migrants who have settled in the UK can take part in citizenship ceremonies

Most migrant voters come from established Commonwealth communities - India, Pakistan, Bangladesh, Nigeria and South Africa - as well as the Irish Republic.

I am the first to accept that people don't necessarily move to this country and immediately think of voting Conservative Grant Shapps, Conservative Party chairman

In contrast, the report said, European Union nationals living in the UK will be "heavily under-represented" because a large majority have not acquired British citizenship.

The report highlighted a challenge for the Conservatives, particularly over migrant voters.

At the 2010 election, 16% of black and ethnic minority voters chose the Tories; 68% voted Labour.

Generations of migrants had formed an image of Labour as the party that "protects migrant and minority interests, in contrast to the Conservatives", the study said.

Conservative chairman Grant Shapps acknowledged there was a "big challenge" ahead for his party, but stressed "things were changing".

"I am the first to accept that people don't necessarily move to this country and immediately think of voting Conservative.

"That's the experience of my own family. I am third-generation British and my Dad, who was second-generation, carried a Labour Party card," Mr Shapps said.

"It wasn't until later in his life that he thought: 'The party of aspiration, the party backing people who are working hard and trying to get on in life are the Conservatives.'"