Most Tory voters don’t believe that Boris Johnson’s Brexit will lead to Scottish independence and the break up of the UK, a new poll has found.

The YouGov survey shows that 70% of Conservatives think leaving the EU will improve the union or make no difference, despite renewed fears of a Scottish breakaway or Irish unification. Just 30% think it will weaken bonds between the UK’s four nations.

But the poll, commissioned by Southampton University and shared with HuffPost UK, also found that voters who see themselves as “English” rather than “British” are more likely to vote for Johnson – and many think Brexit will make England stronger.

Labour and Liberal Democrat supporters are more likely to class themselves as “British” rather than “English” and to believe Brexit will damage both England and the union.

The largest group of voters (40%) are ‘equally English and British’ and their votes are fairly even shared between the parties, but there are clear differences between the way English and British identifiers intend to vote and what they think Brexit will mean for the UK.

Most Conservatives do not believe that the union between England, Wales, Scotland and Northern Ireland will be seriously damaged by Brexit. 18% of Tory voters think Brexit will make the union stronger, and 41% that it will make no difference.

While 30% of Conservative voters think Brexit will weaken the union, only 5% think it will make the union ‘a lot weaker’. 25% say it will make the union ‘a bit weaker’.

Some 78% of Labour and 87% of Lib Dem voters think that the union will be damaged by Brexit, and believe (57% Labour and 62% LibDem) it will be ‘a lot weaker’.

Johnson has repeatedly ruled out agreeing to any new referendum on Scottish independence should he stay as PM, even though the SNP’s Nicola Sturgeon has said her party will have a clear mandate for one after any Brexit.

With the DUP detached from the Tories, several commentators have also suggested that calls for a new “border poll” on Irish independence will grow if Brexit harms Northern Ireland’s economy.

John Denham, former Labour minister and now Director of the Centre for English Politics and Identity at the University of Southampton, said the trends were likely to play a key role in the general election.

Conservative support is much more optimistic about England’s future and much less pessimistic about Brexit than the electorate as whole,” he said.

“This may help the party in Leave voting marginals where voters are more likely to identify as English but be out of step with voters in some Conservative-held Remain seats.”

Johnson’s lead in the opinion polls is heavily weighted towards voters who emphasise their English identity.

The Conservatives are getting 40% of their support from voters who say they are English not British (22%) or ’more English than British (18%), although these voters only make up 28% of the English electorate.

By contrast, Labour and the Lib Dems are promised more support by those who are more British. These voters are just 20% of the electorate but make up 27% of Labour’s vote and 40% of the LibDems.

Labour and the LibDems both get around 10% of their votes from voters who identify the nationality as ‘other’ than British or English – a figure that is just a tiny 2% for the Tories.

Some 75% Conservatives supporters overwhelmingly believe that leaving the EU as currently planned will ‘make England stronger’. Buy contrast Labour (77%) and Liberal Democrats (94%) think Brexit will ‘make England weaker’.

“We see a strong correlation between voters’ national identity, political preferences and world view, particularly amongst those who emphasise either their English or British identity,” Denham said.

“Boris Johnson’s clear poll lead comes from voters who emphasise their English identity. They believe that Brexit will strengthen England, and but do not believe that leaving the EU will do serious harm to the union.

Political arguments about the status of Northern Ireland and demands for a second Scottish referendum do not appear to have made an impact on the perceptions of Conservative supporters.

“Although earlier polls did suggest that Leavers would be happy to see the union break up in order to get Brexit, it seems that most of those planning to vote Conservative in England are simply not convinced that the union will be made significantly weaker.”

YouGov interviewed 1,472 people between December 2 and December 3.