A hung parliament is much more likely at the next election as proposed boundary changes will reduce the number of Labour MPs, say researchers.
There are plans to reshape 500 of 573 constituencies in England and Wales to reflect population changes.
If changes had been in place during the last general election, Labour's 66-seat majority would have been cut to 48, say University of Plymouth researchers.
Then a swing of just 1.6% could have lost Labour its overall majority.
Parliamentary boundaries are reviewed about every 10 years to ensure each constituency has roughly the same number of potential voters.
The research suggests Labour would lost seven seats, while the Tories would gain 12. The number of seats for the Lib Dems would remain the same.
In Labour-held Sittingbourne and Sheppey, the proposed changes would make the constituency more likely to go Conservative at the next election, based on votes polled at the last general election.
Labour councillor Roger Truelove told the BBC: "We are going to have to fight, as this comes into the constituency we do need to work it. "
Conservative challenger Gordon Henderson said: "It's important that we have as many people in our constituency that are inclined to vote Conservative obviously, but we won't take anything for granted."
While the changes could mean Labour's majority is reduced, the researchers estimate the Conservatives would need to achieve one of their biggest swings in votes since 1931 to achieve a majority of their own.
Professor Colin Rallings, of Plymouth University, said: "Perhaps the most interesting aspect of these boundary changes is that they reinforce the idea that a hung parliament is really quite a likely possibility at the next election.
"There's a very wide range of swing from Labour to the Conservatives which would produce a parliament with no party with an overall majority and, of course, that would mean that parties would have to do deals and trade with others to try to form a government."
The changes will increase the number of seats in Parliament from 646 to 650, with all the new posts being created in England.
The large northern cities, and London, have lost seats to the rural south of England because of its population growth.
The alterations have been approved for Wales, but not yet for England.