The Liberal Democrat leadership race is on a knife-edge with postal votes cast splitting “almost 50-50” between Jo Swinson and Sir Ed Davey, The Independent has learned.

The figures were described as “a major surprise” by a source at the party’s headquarters – given Ms Swinson has been the odds-on favourite to win the contest on Monday.

One supporter of Sir Ed’s campaign said he was now in a strong position, arguing he was gaining an edge the longer the race to succeed Vince Cable has progressed. “It really is game on now,” the backer said.

No votes have yet been counted in the contest, ahead of it closing at midday on Monday, but postal ballots have been checked to ensure no one has also voted electronically.

It is these slips which have divided almost equally between Sir Ed, the former energy secretary, and Ms Swinson, a business minister in the coalition government, The Independent has been told.

The Lib Dem source said: “Postal ballots were counted and verified on Friday. There was a major surprise as it showed an almost exact 50-50 split, although Swinson is the clear bookies favourite.”

Around 60 per cent of the Lib Dems’ 107,000 members had voted at this point. However, only a minority of votes were by post – and therefore split down the middle.

Nevertheless, if the postal returns are matched by votes cast electronically it points to a much closer race than expected at the outset, when Ms Swinson was the clear frontrunner.

Although little divides the two candidates on policy, critics say she has appeared more hesitant in the many hustings, including a live BBC TV debate on Friday.

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after stopping in Sutton, Surrey on the last day of the general election campaign Paddy Ashdown – Former Liberal Democrat leader in pictures 1997 Leader of the Liberal Democrats Paddy Ashdown gives his keynote speech to the party's annual conference in Eastbourne REUTERS Paddy Ashdown – Former Liberal Democrat leader in pictures 1998 Then Liberal Democrat leader Paddy Ashdown and his wife Jane acknowledging the crowd following his speech at the Liberal Democrats conference in Brighton PA Paddy Ashdown – Former Liberal Democrat leader in pictures 1999 Paddy Ashdown in his London office on the day that he handed in his formal letter of resignation from the position of Party Leader of the Liberal Democrats PA Paddy Ashdown – Former Liberal Democrat leader in pictures 2000 Paddy Ashdown with his wife Jane and dog, Luke, at their home at Norton sub Hamdon PA Paddy Ashdown – Former Liberal Democrat leader in pictures 2007 Former British Liberal Democrat Party leaders David Steel, (left) 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party's spring conference in Liverpool Reuters Paddy Ashdown – Former Liberal Democrat leader in pictures 2016 Then Prime Minister David Cameron helping to campaign for a 'Remain' vote in the EU referendum at a phone centre in London along with fellow pro EU campaigners, Lord Ashdown (left) and Lord Kinnock PA Paddy Ashdown – Former Liberal Democrat leader in pictures 2016 British Prime Minister David Cameron listens to Former Liberal Democrats leader Paddy Ashdown as he addresses students and pro-EU 'Vote Remain' supporters during his final campaign speech at Birmingham University Getty Images

Sir Ed is warmer towards pacts with other parties, a Lib Dem flashpoint after the price paid for the deal with David Cameron, advocating an anti-Brexit government of national unity under a centrist Labour leader.

His opponent has called for local pacts to allow a single pro-Final Say referendum candidate a clean run in marginal constituencies, to maximise representation in the Commons.

Although the race has been overshadowed inevitably by the simultaneous contest to lead the Conservative party – and the country – the winner could yet have a crucial role in the Brexit drama.

Sir Vince will bow out on a high, after his party’s remarkable recovery at the May local and European parliament elections, made it unquestionably the leading Remain force.

And the new leader is likely to enjoy an early present with victory in the Brecon and Radnorshire by-election on 1 August, slashing the Tory majority to just three.

A source on Sir Ed’s team said, of the knife-edge voting: “A lot of these postal votes would have been sent in early in the campaign when Jo was getting most of her coverage.

“Since then, Ed has been getting much more in the media and has got the better of Jo on policy and substance in hustings up and down the country. It really is game on now.”