Boris Johnson could have won by an even bigger majority after Nigel Farage’s Brexit Party blocked 20 seats.
Internal Conservative figures suggest the party could have grabbed even more seats if the Brexit party had not been standing.
Mr Farages decision to pull out of Tory-held constituencies was key to the campaign, allowing the Tories to unite the Leave vote.
The Prime Minister captured 364 of the 650 seats, winning a comfortable majority of 74.
Despite winning a stonking majority, the stunning figures show he could have smashed Margaret Thatcher’s record of 102 if Mr Farages group had not stood at all.
Despite running in seats all over the country, the Brexit Party failed to land a single one.
Paul Hilder, chief executive of the polling firm Datapraxis, said: According to our analysis, there are at least 20 Labour-held seats where the Brexit Party likely cost the Tories victory.
One of the things that happened through the campaign was that some of the Labour to Brexit Party switchers came back to Labour, a few went tactically to the Tories and some ended up staying home.
In seats such as Hartlepool, Rotherham and Barnsley Central and East, between 70 and 90 per cent of Brexit Party voters said they would vote Conservative if it was a two-horse race, with a maximum of 6.5 per cent choosing Labour instead.
In Hartlepool, one in four voters backed the Brexit Party chairman Richard Tice, allowing Labours Mike Hill to cling on with a majority of 3,595.
In Rotherham Sarah Champion won with a majority of 3,121 as the Brexit Party raked in 6,125 votes.
In Barnsley Central, former soldier Dan Jarvis retained his seat by less than 4000 as Mr Farages candidate came second with 11,233 votes.
Former Labour leader Ed Miliband could have also gone, winning in Doncaster North by 2,370 as the Brexit party won 8,924.
In nearby Doncaster central, former Labour chief whip Dame Rosie Winterton also got away with it as the Brexit party got more than double her 2,278 majority.
Yvette Cooper also barely survived, with the leadership contender holding on by 1,276 in Normanton, while the Brexit Party took 8,032 votes.
Other pollsters also backed the analysis, claiming even more of Labours Red Wall could have fallen.
Pippa Norris, of Harvard University, wrote: My estimates suggest that the share of the Brexit vote was large enough to allow the Conservatives to slip in the back door and make up to 20 seat gains in former Labour areas.
