The survey, carried out for the Sun, showed that almost half of those who backed Ukip at the last General Election are now planning to vote for the Conservatives on Thursday.
The Prime Minister is expected to snatch millions of votes from former Ukip supporters
In February, just 20 per cent of those who previously backed the anti-EU party in 2015 pledged to vote for the Conservatives.
The news that 48 per cent of Ukip voters are set to back Theresa May will come as welcome news for Tories strategists, who have seen the party’s lead over Labour shrink rapidly in recent weeks.
Just 23 per cent of Ukip’s 3.9 million voters in 2015 say they will vote for the party this time, while 11 per cent will defect to Labour and 2 per cent will back the Lib Dems.
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Jeremy Corbyn during a visit to Hackney Marshes Football Pitches, to highlight Labour's manifesto commitment to ensure 5% of the Premier League's television rights income is diverted to the grassroots game, during a General Election campaign
Around 35 per cent are yet to make up their mind, or have said they may change their vote before polling day on Thursday.
ComRes Chairman Andrew Hawkins said: “Theresa May’s appeal to former Ukip voters is worth almost 6 points on her poll lead over Labour, which might make all the difference between a workable majority and a landslide on Thursday.”
Mrs May’s hopes of gaining a bigger majority are boosted by the fact that 79 per cent of Ukip voters trust the Prime Minister to deliver a good Brexit deal, compared with just 54 per cent for party leader Paul Nuttall.
Meanwhile, Ukip voters have shown a notable dislike for Jeremy Corbyn and Labour, with 69 per cent of those polled claiming they don’t like the leader or the party.
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Ukip won almost 4 million votes at the 2015 General Election
Speaking after Saturday’s devastating attacks in central London, Ukip leader Paul Nuttall said he “wouldn’t take anything off the table” in order to keep Britons safe from an imminent terror attack.
During a grilling on BBC’s Election Questions, the Ukip leader repeated calls for headscarves to be banned in public spaces, internment to be considered for terror suspects and waterboarding to be reinstated where it could prevent an incident.
On his proposals for waterboarding to be permitted in certain cases, the MEP said: “I mentioned waterboarding in the case if there was going to be an imminent terrorist attack, and if we had to find out information quickly, I wouldn't take anything off the table to ensure British women, men and children are kept safe.
Theresa May campaigning in Bradford on Monday
"When are we going to realise that there's a group of people in this country – MI5 say there's 23,000 – who hate who we are, hate how we live and want to kill us?
"What I want to do is ensure that things like Manchester and London last night do not become common in this country."
Mr Nuttall also welcomed Theresa May’s declaration that “enough is enough” on Islamist extremism in the UK, and said it was time mainstream politicians stopped “turning a blind eye” to what he described as the “cancer” of Islamist ideology.