More than one million Commonwealth immigrants - who have not qualified for British citizenship - could vote in the next general election, campaigners have warned.
Commonwealth citizens - from 54 independent sovereign states, including Australia, Canada and India - can register on the electoral roll upon arrival in the UK as long as they have an address.
Campaigner Migration Watch UK estimates that by the next general election in 2015 there will be more than one million Commonwealth citizens who do not have British citizenship with the right to vote.
And, except for a few Caribbean states, British citizens do not have similar rights in Commonwealth countries, the group added.
Migration Watch UK chair Sir Andrew Green added: "If the next election is close, and especially if there is a coalition negotiation, the number of seats gained by each party will be critical, so the outcome could be significantly influenced by a group of people who have not yet qualified to become British citizens or have not even bothered to do so.
"One example is that a student visitor from a Commonwealth country planning to study for six months would be allowed to enrol on the electoral register and vote in a parliamentary election if one was held during the duration of his or her stay.
"This is clearly absurd. The issue is one of basic fairness. If people want to participate in deciding the future of our country they should at least become citizens."
The 2011 census data shows that there are 960,000 Commonwealth citizens who do not have British citizenship living in England and Wales but who have the right to vote. This total includes only those who, in 2011, were over the age of 15 and who therefore would be eligible to vote in 2015, Migration Watch UK said.
If the turnout is close to the national average, this could amount to more than half a million votes, the campaigners added.
A Cabinet Office spokesman said: "The right to vote in UK elections for Commonwealth citizens who live here reflects our close historical ties with Commonwealth countries. Excluding Commonwealth citizens would be a significant step and would require careful consideration. Parliament has previously taken the view that these rights should not be changed."