BBC audiences may have been hijacked by Jeremy Corbyn supporters posing as Tories, polling experts have warned.

It follows an admission from the head of polling company ComRes, which gathered the audience for a seven-way debate earlier this week, that Amber Rudd and the Ukip leader Paul Nuttall faced a tougher time than their left-wing opponents at the hands of the audience.

Senior polling experts have admitted privately that they are aware of groups of Labour activists, some led by grassroots Corbyn support group Momentum, encouraging his supporters to sign up for political debates posing as Conservatives in order to make it look like Labour has won the debate.

The problem occurs when polling companies or broadcasters recruit audience members using an open online form, which allows people to sign up repeatedly using different email addresses and is more open to abuse, according to experts.