Three leading campaigners for a second Brexit referendum have been refused passes for the Conservative party conference, prompting them to complain that the governing party is suppressing voices it disagreed with.

Eloise Todd, Best for Britain’s director, was among those who were refused passes on Thursday night in a terse email that gave no explanation as to why her accreditation and that of two colleagues was not granted. Russian diplomats have also been refused passes.

“The Conservative party can put their heads in the sand but it doesn’t change the fundamental and unavoidable truth that public opinion is shifting away from Brexit,” Todd said. “A party of government should always be listening – even to voices it may disagree with.”

The email from the Conservatives informed Todd that “having considered your application for a security pass to attend the Conservative party Conference, I write to inform you that your application for accreditation has not been granted”.

Best for Britain will hold a fringe event at Birmingham outside the secure perimeter, with speakers including Phillip Lee, a Conservative junior minister who resigned over the government’s Brexit policy. The group also plans to buy a wraparound advert in the Birmingham Mail and take out billboards to remind Conservative delegates of their campaign.

Theresa May has repeatedly ruled out holding a second referendum on a final Brexit deal, despite suggestions there is support for the move among senior Tory remainers. On Thursday more than 100 constituency Labour parties submitted motions calling for Labour to back a second referendum.

The Russian embassy in London claimed on Friday its diplomats had been denied credentials to attend the Tory conference.

A London embassy source told the Russian news agency Tass: “Over many long years, members of the embassy staff have traditionally attended the conferences of the Conservative party, the Labour party and sometimes the Liberal Democratic [arty regardless of the state of Russian-British bilateral relations, but the Tories denied registration to us this year.

“We were given to understand the presence of official Russian representatives at the conference would be undesirable,” the source said.

He described the annual conferences of the UK’s leading parties as “important events in the country’s life, in the course of which the largest political forces map out their priorities on a broad spectrum of issues and hold open and frank discussions on the pressing problems of domestic and international politics.

“In addition, they [the conferences] offer the floors for contacts between leaders and party activists with representatives of the diplomatic community,” he said.

On Thursday Downing Street accused Russia of acting with contempt and allowing “lies and blatant fabrications” to be aired on a state-funded broadcaster, after an extraordinary interview with the two men identified as suspects in the Salisbury nerve agent attack was broadcast on Russia Today.

The pair, who identified themselves as Alexander Petrov and Ruslan Boshirov, claimed they had been in Salisbury to visit the cathedral.