Boris Johnson must appear in court to face allegations of lying to the British public during the 2016 Brexit referendum campaign, a judge has said.

The Conservative leadership frontrunner was summonsed after a campaigner brought a private prosecution over claims that EU membership cost the UK £350m a week.

The frontrunner in the Tory leadership race has been accused of misconduct in public office after making the claim during the campaign, following a private prosecution by campaigner Marcus Ball.

Tory Brexiteer Jacob Rees-Mogg said the decision was a “troubling” abuse of process.

He told the Press Association: “It is trying to use the courts to achieve a political end which, I think, is neither right or proper. This is involving the courts in something that is not their area.

“We need courts and politicians to respect each other, and it is an abuse of process, and a troubling one. It has been brought by people who are resentful of the referendum result.”

And Conservative former cabinet minister and barrister David Mellor said the ruling was a “deplorable absurdity”, and that courts should not adjudicate on what politicians do during election campaigns.

“I imagine there will be no shortage of senior judges who will feel acutely embarrassed about this,” he told the Press Association.

“Politicians at election times exaggerate, and say things that may or may not be true, and it’s the electorate, not the courts, who should decide whether they are reliable or not.

“This is a bad day for British justice. But probably, contrary to the wishes of those who have crowdfunded this nonsense, a big boost to Boris. Is that what they really intended? Nutty, nutty, nutty.”

Health Secretary Matt Hancock, who is also vying for the top job, said: “However people voted in the referendum, we shouldn’t have courts judging on political debates.

“Let’s have robust debate to test arguments – and keep courts out of politics.”

It came after the EU slapped down Tory leadership hopefuls aiming to alter the Brexit deal, with European Commission president Jean-Claude Juncker warning the bloc was “crystal clear” that “there will be no re-negotiation”.