Some campaign groups are suggesting people use tactical-voting websites, to help them decide which candidate to support. One website has faced criticism over the nature of its advice.

But what is tactical voting and is there any evidence it works?

What is tactical voting?

Put simply, tactical voting is when someone backs a candidate they wouldn't normally support, to stop someone else winning.

This could happen in a constituency where two parties are in a tight race and candidates from other parties trail far behind.

In these circumstances, a supporter of the candidate who was a distant third, might pick their favourite of the two who are in with a chance.

At this election, campaigners say voting tactically could help MPs who share voters' views on Brexit win more seats.

Why would tactical voting matter in the UK?

In a general election, the UK's 46 million voters are invited to choose an MP for their area - one of 650 constituencies.

The UK uses a first-past-the-post voting system, sometimes described as "winner takes all".

It means the local candidate who receives the most votes becomes an MP - and the party with the most winning candidates nationally normally forms a government.

Am I allowed to vote tactically?

Tactical voting is perfectly legal in the UK. Voters are free to choose any candidate on the ballot paper, no matter what the reason is.

Parties themselves sometimes make tactical decisions not to stand in certain seats.

This is to prevent the vote splitting, which could prevent another party with similar political views being elected.

In August, for example, the Liberal Democrats beat the Conservatives in the Brecon and Radnorshire by-election, after Plaid Cymru and the Greens agreed not to stand.

At this election, the Brexit Party has said it could be open to an electoral pact with the Conservatives. It believes this could increase the chances of a Brexit-supporting government winning power.

Will tactical voting increase at this election?

This election is going to be highly unpredictable, researchers at the British Election Study say.

That's because voters could be more likely to switch their support, largely because of the uncertainty over Brexit.

Last month, the Electoral Reform Society campaign group asked polling company BMG Research to find out how widespread tactical voting might be.

Of 1,500 voters questioned, 24% said they planned to vote tactically to keep out a candidate they dislike.

That compares with 66% who said they would vote for their first preference - regardless of how likely they were to win. The remaining 10% said they didn't know.

When the same question was asked before the 2017 general election, 20% of people said they planned to vote tactically.

Is there any evidence tactical voting works?

Tactical voting has been a very powerful tool in the past, says Stephen Fisher, professor of political sociology at the University of Oxford.

"It played a big role in delivering a landslide for Labour's Tony Blair in 1997 and it's been a staple of elections since then," he says.