Migrant voters could have a “decisive” impact in marginal seats in May’s general election, according to a new study co-authored by the University of Manchester and Migrants Rights Network.

The findings, published yesterday, will make uncomfortable reading for those parties that have exploited fears over immigration as a key campaigning plank.

Almost four million foreign-born voters in England and Wales will be eligible to cast a vote on May 7.

The report suggests that for the first time in a general election, MPs will be returned from two constituencies — East Ham and Brent North — where the majority of voters were born overseas.

“Foreign-born residents of the UK could have an immediate impact in the May 2015 general election,” the report said.

“Not only could migrant voters comprise a significant number of overall potential voters on May 7 but they could turn out in substantial numbers within some key marginal constituencies.”

Migrants could constitute more than a third of voters in about 25 seats in England and Wales and at least a quarter of the electorate in more than 50 seats, researchers found.

In at least 70 seats the migrant share of the electorate will be double the majority of the current MP.

The report warned that parties which concentrate on voter concerns about immigration could be alienating an important section of the electorate — potentially for years to come.

“The risk for politicians today is that focusing primarily on the anxieties of those native voters with very negative views about immigration could alienate this new migrant electorate,” the report said.