Sir John Curtice told Express.co.uk the Conservative vote fell “mainly among voters on leave”. He explained Leave voters are traditionally less loyal to the Tory party as many had never voted Conservative before the 2016 referendum. This comes after Mr Johnson’s party suffered a deadly election result local last week.
The party lost nearly 500 seats and lost control of 11 councils across the UK.
He gave his support mainly to the Liberal Democrats in the south of England and lost key advice from London to Labour.
The party also took a beating in Scotland and Wales, where the SNP and Labor won seats respectively.
Sir John said many of those losses came from people who voted on leave in 2016.
He said this group of people had been “pushed” by damaging events such as Partygate, the cost of living crisis and the Covid-19 pandemic.
Speaking to Express.co.uk about the election results, Sir John said: “It’s because the Stay/Leave divide has narrowed so much.
“If you look at the opinion polls, Labor has gained a lot more ground among incumbents than among remaining voters.
“The Conservatives have lost a barrel of Leave voters.
“Therefore, these people are going to be more fragile.
“When the government was struggling with Covid in the fall of 2020, it was among absentee voters that its vote dipped.
“And Partygate and the cost of living crisis pushed it.
“Essentially, it has to do with the fact that these are the people who have to be constantly persuaded that voting Conservative is a good idea.
“It turns out to be difficult to do.”
Many Brexit strongholds, such as Rochford and South Tyneside, turned their backs on the Conservative Party in local elections.
In Rochford, which saw 60% of people vote to leave the EU in 2016, the Conservative party lost six seats.
In their place, the Lib Dems won two seats and the independents four.
Meanwhile, in South Tyneside, Labor held the seat, with the Green party winning three seats.
In 2016, South Tyneside was a Brexit stronghold, with 62.0% of the vote going to the Party.