Migrant voters could have a "decisive" impact in a range of key marginal seats in the forthcoming general election, a new study has found.

Almost four million foreign-born voters in England and Wales will be eligible to cast a vote on May 7, according to a report by academics at the University of Manchester and the Migrants' Rights Network.

Their findings suggest that for the first time in a general election, MPs could be returned from two constituencies - East Ham and Brent North - where the majority of the electorates is 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."

The report said that migrants could constitute more than a third of the voters in around 25 seats in England and Wales and at least a quarter of the electorate in more than 50 seats, the report said.

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

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

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

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.

It said that generations of migrants of had had their view of the Conservative Party shaped by the hostility of Enoch Powell and his supporters while Labour was seen as the party which protected minority interests.

"Politicians who are keenly attuned to the concerns of voters worried by migration have often been rather less sensitive to the concerns of the migrants whose rights and security are threatened by reforms promising restrictions to freedom of movement, family reunion and access to welfare assistance," the report said.

"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.

"Persistent hostility or indifference from sections of the political class could encourage the second wave of migrants to form a settled image of such parties as inherently opposed to their interests, just as the first did."