Safe seats 'a sorry indictment'
Campaigners for a more proportional voting system said the result in well over half of constituencies could be taken as read, almost four weeks before voters go to the polls.
The Electoral Reform Society said it could confidently predict the winner in 364 "safe" seats - 56% of the total 650 - because the UK's first-past-the-post arrangement meant resources were targeted at marginals.
As many as 25 million voters were being ignored, it said, with the average seat not having changed hands since the 1960s and some not since Victorian times.
Katie Ghose, chief executive of the ERS which has developed an online tool for voters to find out if they live in a "safe" seat, said: "The fact that we can firmly predict the outcome of over half of the seats being contested this May is a sorry indictment of our outdated voting system.
"The average constituency hasn't changed hands since the 1960s, and some have been under the same party's control since the reign of Queen Victoria.
"This is a huge disincentive for people to get out there and vote, and for other parties to challenge incumbents. We have a system which actively discourages voters and parties from taking part.
"We urgently need a fairer electoral system to give voice to the many millions of people in safe seats who are not being heard."
"Predicting the winners in a majority of seats shouldn't be possible in a truly dynamic and modern democracy. But sadly that's exactly what we've been able to do. The UK needs a fair voting system to bring our politics into the 21st century and give us a Parliament that actually reflects people's wishes."
It said it could predict the result in 186 of 303 Tory-held seats, 150 of 257 Labour and seven of 56 Liberal Democrat.
The areas with the most "safe" seats were North East England (79.3%), Northern Ireland (77.8%), East of England (70%), South East England (69%) and London (68.5%).
"As the loss of safe seats is rare, parties target resources on a small number of floating voters in marginal seats - meaning they give up on millions of voters across the country," the ERS said.