World at One host Martha Kearney apologises for the language used by Pimlico Plumbers boss Charlie Mullins

The BBC has apologised after Jeremy Corbyn was called a “twat” in a Radio 4 interview by Charlie Mullins, the boss of Pimlico Plumbers and a Conservative party donor.

Mullins was being interviewed on Radio 4’s World at One when host Martha Kearney asked whether the UK should attempt to remain part of the single market after Brexit.

Mullins replied: “Course we should be staying in; Jeremy Corbyn’s a twat, ain’t he?”

Kearney then apologised to the Labour leader for the comment, saying: “Well, we’ll definitely leave it there. Apologies to Jeremy Corbyn for the language used.”

The Pimlico Plumbers boss was referring to Corbyn’s stance that the UK should leave the single market if it leaves the EU. Corbyn told the BBC’s Andrew Marr Show last weekend that membership of the single market and the EU were “inextricably linked”.

Mullins apologised after the show for swearing “but not the sentiment behind it”. He said on Twitter: “Brexit is a subject I’m very passionate about and I apologise for the word I used on BBC World at One, but not the sentiment behind it.”

A Labour source said: “Jeremy condemns all, and does not engage in any, personal abuse.”

Mullins appeared on the World at One to comment on a ruling by the supreme court that has caused the government to pledge that employment tribunals will be free.

The UK’s highest court came down in favour of the trade union Unison, which argued that fees of up to £1,200 were preventing workers – especially those on lower incomes – from getting justice.

Pimlico Plumbers, which has donated more than £48,000 to the Conservative party in the last two years, lost a separate case earlier this year against a plumber who challenged the company’s view that he was self-employed at an employment tribunal.

Mullins told the radio programme that the scrapping of fees for employment tribunals was a “ridiculous, stupid, dangerous and irrational decision” and would lead to “malicious” claims from employees.