Pollsters often struggle to predict a Tory majority in referendums and elections because Conservative voters are busier and far more difficult to get hold of, eminent political scientist John Curtice has said.

Prof Curtice, whose exit polls correctly predicted the 2015 general election when others had failed, admitted that Labour voters were more likely to be at home - rather than out at work or undertaking social engagements - when samplers phoned or called round.

In recent years polling companies have been criticised for forecasting results incorrectly.

To find out if availability was having an impact on polling, Prof Curtice looked at answers given in the British Social Attitudes Survey 2015, which records voting preference. He found that if polls had been based on the people who answered the door on a first visit then Labour would be six points ahead.

In contrast, if polls were based on those who needed three to six visits before answering then the Tories had an 11 point lead.

Prof Curtice said it wasn’t ‘shy Tories’ who skewed polling results but ‘busy’ ones.

“The people who the pollers interviewed are not always fully representative. It wasn’t that people were lying, or saying they would vote and then not turning up” he told a briefing in London.