Confusion about tactical voting could cost pro-remain election candidates victory and lead to a Conservative government winning a working majority, according to the political campaigner Gina Miller.

The founder of Remain United, which advocates tactical voting as a way to halt Brexit, said the volatility of the electorate was at an unprecedented level.

The failure of Labour and the Liberal Democrats to form alliances risked the Conservative party securing enough MPs to push through the Brexit deal, she said.

Canterbury is among the top seats where remain voters are being urged to back Labour candidate Rosie Duffield, ahead of the Liberal Democrat candidate Claire Malcomson.

She was selected at the 11th hour after the original candidate Tim Walker stood aside.

Mrs Miller, who took the government to court to force it to consult parliament about its Brexit deal, said it had been a mistake for the Liberal Democrats to helicopter a candidate into Canterbury.

But she said the greatest threat was the proliferation of tactical voting websites, which often give conflicting views about how to vote.

"In 2017, we were the only real tactical voting website," said Mrs Miller.

"The 6.5m who voted tactically in 2017 did the trick of stopping a Tory majority; it wasn’t the only reason but it contributed significantly to that outcome.

"Now we have more challenges; tactical voting was not at the front of peoples' minds in 2017; our research suggests 40% of voters on the remain or soft leave side are willing to consider tactical voting.

"I am not saying they all will but it is much higher than it was."

Mrs Miller rejected the suggestion the Conservatives had been given an advantage by the Brexit Party, which is standing down in 317 seats they hold.

However, she acknowledged Remain United had, as a result, projected a smaller number of marginal seats where tactical voting could make the difference.

"In some marginals, where it really does matter, the remain vote will be split between Labour and the Lib Dems.

"Somebody in my view has to decide whether there are going to be 'paper' candidates or else they will lose the battle.

"We were at 68 marginal seats but that dropped to 51 so it did have an impact but it was not catastrophic.

"From a tactical point of view, there is still time for us to succeed and to reach the people we need to reach; we cannot convince everyone in the time we have; we don’t have the resources."

On the pact agreed by the Liberal Democrats and the Green Party she said it was not significant. "It will only make a difference in three or four seats because Labour is not involved."

The agreement has led to the Green Party withdrawing from one seat in the county, Tunbridge Wells to help the Liberal Democrat candidate.

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