The survey of 2,016 people found a third of those who voted Ukip in 2015 will be voting tactically, while 6 per cent of those who backed the SNP will be holding their nose and voting to keep another candidate out.
The research, by the Electoral Reform Society, said 9 per cent of those polled in the run-up to the last election voted for a "lesser evil".
Darren Hughes, deputy chief executive of the society which promotes electoral reform, said: "It is frankly astonishing that a fifth of Brits feel unable to vote for their first choice party this election.
It is frankly astonishing that a fifth of Brits feel unable to vote for their first choice party
Darren Hughes, Electoral Reform Society
"That's a huge proportion of people having to hold their nose and opt for a 'lesser evil' rather than who they actually support – and a significant and worrying rise on the last election.
"This whole situation turns elections into a gamble around splitting the vote and trying to predict who on the left/right is most likely to win.
"That isn't a democratic election – it's a lottery election."
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He added that proportional systems – as used in Scotland and Northern Ireland – should be instated for Westminster elections.
He said: "Rather than simply throwing people's votes away because they haven't been 'tactical' enough, it's time for a system where every vote counts.
"No one should feel unable to vote for their favourite party – and this should be the last election where that's the case."
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