Grassroots Tories pushing for a fresh Brexit referendum have attacked their party’s “intolerance”, after a conference debate on the campaign was obstructed.

The Conservatives have been accused of an “extraordinary” attempt to undermine discussion on the most crucial issue facing Britain, by refusing to allow a meeting to publicised.

In an interview with The Independent, the head of the group Conservatives for a People’s Vote said the stance was part of a shift to the right as Theresa May tried to carry out her form of Brexit.

And he made the explosive claim that the situation was similar to “what happened in the Labour party with Militant in the 1980s” – the benchmark for an attempt by extremists to take over a major party.

Simon Allison, the group’s chairman, said: “The party is going in the direction of intolerance and there is a clear move to shut down debate on the most important issue of the day.”

And he warned: “There is significant infiltration of the party from Ukip and pro-EU MPs are being threatened with deselection and face online trolling, which is not unlike what is happened in the Labour party with Militant in the 1980s.

“Do they really want to drive moderates out of the party entirely? Because we are not going anywhere, we will keep fighting.”

Mr Allison, a former Tory researcher and parliamentary candidate, spoke out after Conservatives for a People’s Vote was barred from publishing an entry in the official programme at the Birmingham conference.

The block was imposed despite planned speeches by a former cabinet minister, Justine Greening, and two other prominent Conservatives, Phillip Lee and Sarah Wollaston.

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Tens of thousands of people demonstrated in London on Saturday calling for a second vote on Britain's departure from the European Union. / AFP PHOTO / Niklas HALLE'NNIKLAS HALLE'N/AFP/Getty Images NIKLAS HALLE'N AFP/Getty Images People's Vote march – demanding vote on final Brexit deal A young protestor shouts as she takes part in the People's Vote demonstration against Brexit Getty Images People's Vote march – demanding vote on final Brexit deal A woman holds a placard as she joins EU supporters, calling on the government to give Britons a vote on the final Brexit deal, participating in the 'People's Vote' march in central London, Britain June 23, 2018. REUTERS/Henry Nicholls HENRY NICHOLLS Reuters People's Vote march – demanding vote on final Brexit deal epa06833888 Pro-EU demonstrators take part in a People's March anti-Brexit rally in Parliament Square in London, Britain, 23 June 2018. Protesters are calling for a referendum on the final deal. The protest is taking place on the second anniversary of the Brexit referendum. EPA/ANDY RAIN ANDY RAIN EPA People's Vote march – demanding vote on final Brexit deal A protester's pro-EU t-shirt EPA People's Vote march – demanding vote on final Brexit deal epa06833892 A pro-EU demonstrator waves a European Flag during a People's March anti-Brexit rally in Parliament Square in London, Britain, 23 June 2018. Protesters are calling for a referendum on the final deal. The protest is taking place on the second anniversary of the Brexit referendum. EPA/ANDY RAIN ANDY RAIN EPA People's Vote march – demanding vote on final Brexit deal Demonstrators carry banners and flags as they participate in the People's March demanding a People's Vote on the final Brexit deal, in central London on June 23, 2018, the second anniversary of the 2016 referendum. Tens of thousands of people demonstrated in London on Saturday calling for a second vote on Britain's departure from the European Union. / AFP PHOTO / Niklas HALLE'NNIKLAS HALLE'N/AFP/Getty Images NIKLAS HALLE'N AFP/Getty Images People's Vote march – demanding vote on final Brexit deal epa06833960 Tens of thousands of people march through London during a People's March anti-Brexit demonstration in London, Britain, 23 June 2018. Protesters are calling for a referendum on the final deal. The protest is taking place on the second anniversary of the Brexit referendum. EPA/ANDY RAIN ANDY RAIN EPA People's Vote march – demanding vote on final Brexit deal Leader of the Liberal Democrats, Vince Cable speaks during the People's March demanding a People's Vote on the final Brexit deal, in central London on June 23, 2018, the second anniversary of the 2016 referendum. Tens of thousands of people demonstrated in London on Saturday calling for a second vote on Britain's departure from the European Union. / AFP PHOTO / Niklas HALLE'NNIKLAS HALLE'N/AFP/Getty Images NIKLAS HALLE'N AFP/Getty People's Vote march – demanding vote on final Brexit deal epa06833964 A young pro-EU demonstrator takes part in a People's March anti-Brexit rally in Parliament Square in London, Britain, 23 June 2018. Protesters are calling for a referendum on the final deal. The protest is taking place on the second anniversary of the Brexit referendum. EPA/ANDY RAIN ANDY RAIN EPA People's Vote march – demanding vote on final Brexit deal Gina Miller and Caroline Lucas EPA People's Vote march – demanding vote on final Brexit deal epa06834044 Tens of thousands of people march through London during a People's March anti-Brexit demonstration, in London, Britain, 23 June 2018. Protesters are calling for a referendum on the final deal. The protest is taking place on the second anniversary of the Brexit referendum. EPA/ANDY RAIN ANDY RAIN EPA People's Vote march – demanding vote on final Brexit deal epa06833264 Thousands of people march through London during a People's March anti Brexit demonstration in London, Britain, 23 June 2018. Protesters are calling for a referendum on the final deal. The protest is taking place on the second anniversary of the Brexit referendum. EPA/ANDY RAIN ANDY RAIN EPA People's Vote march – demanding vote on final Brexit deal Tens of thousands of people march through London EPA People's Vote march – demanding vote on final Brexit deal epa06833265 Thousands of people march through London during a People's March anti Brexit demonstration in London, Britain, 23 June 2018. Protesters are calling for a referendum on the final deal. The protest is taking place on the second anniversary of the Brexit referendum. EPA/ANDY RAIN ANDY RAIN EPA People's Vote march – demanding vote on final Brexit deal Demonstrators at the People's Vote March Getty People's Vote march – demanding vote on final Brexit deal 'Two months too young to decide on my future' REUTERS People's Vote march – demanding vote on final Brexit deal Crowds taking part in the People's Vote march for a second EU referendum at Trafalgar Square in central London. PRESS ASSOCIATION Photo. Picture date: Saturday June 23, 2018. See PA story POLITICS Brexit. Photo credit should read: John Stillwell/PA Wire John Stillwell PA People's Vote march – demanding vote on final Brexit deal A young girl joins in the march PA People's Vote march – demanding vote on final Brexit deal An EU flag is draped across the statue of Winston Chruchill in Parliament Square REUTERS People's Vote march – demanding vote on final Brexit deal Demonstrators gather prior to the start of the People's March demanding a People's Vote on the final Brexit deal, in central London on June 23, 2018, on the second anniversary of the 2016 referendum. / AFP PHOTO / Niklas HALLE'NNIKLAS HALLE'N/AFP/Getty Images NIKLAS HALLE'N AFP/Getty Images People's Vote march – demanding vote on final Brexit deal Vince Cable MP, Pro-EU campaigner Gina Miller, Tony Robinson and Caroline Lucas MP join with crowds PA People's Vote march – demanding vote on final Brexit deal Crowds gather on Pall Mall in central London, during the People's Vote march for a second EU referendum. PRESS ASSOCIATION Photo. Picture date: Saturday June 23, 2018. See PA story POLITICS Brexit. Photo credit should read: John Stillwell/PA Wire John Stillwell PA People's Vote march – demanding vote on final Brexit deal Crowds gather on Pall Mall PA People's Vote march – demanding vote on final Brexit deal A man resembling Britain's Foreign Secretary Boris Johnson, joins EU supporters Reuters People's Vote march – demanding vote on final Brexit deal EU supporters, calling on the government to give Britons a vote on the final Brexit deal, participate in the 'People's Vote' march in central London, Britain June 23, 2018. REUTERS/Henry Nicholls HENRY NICHOLLS Reuters People's Vote march – demanding vote on final Brexit deal People gather in Trafalgar Square REUTERS People's Vote march – demanding vote on final Brexit deal Pro-EU campaigner Gina Miller and Tony Robinson PA People's Vote march – demanding vote on final Brexit deal EU supporters, calling on the government to give Britons a vote on the final Brexit deal, participate in the 'People's Vote' march REUTERS

To add insult to injury, the group submitted a “non-refundable” payment of £360 when it sent in notice of the meeting for inclusion in the programme and on the website.

Dr Lee has written to Brandon Lewis, the Conservative party chairman, demanding to know why he “cannot support free speech” – but has not received a reply.

Mr Allison said: “We have seen, in parliament, the government’s unwillingness to allow a vote on Article 50 and the attempts to stop a meaningful vote by MPs at the end of the negotiations.

“Now there is this attempt to shut down debate on the biggest issue there is at the party conference, by not allowing our event to be included in the handbook or on the website – which is extraordinary.

“They don’t want delegates to go along and listen, yet – when it was party policy to remain in the EU – the hardline Brexiteers staged lots of fringe meetings which were advertised in the handbook.”

Conservatives for a People’s Vote is supported by four Tory MPs, two peers, two MEPs and – Mr Allison said – a growing number of Conservative councillors in a WhatsApp group.

The small businessman argued there was “too much focus on the activists” – who largely favour a hard Brexit – and too little on the very different views of Tory supporters.

Research showed there were 3-4m Conservatives who favoured staying in the EU, many of whom believed another referendum was the only way to resolve the Brexit deadlock.

“My impression from traditional Tory voters – those who are not hardline activists – is very different from many of the activists,” Mr Allison said.

“I hardly meet anyone who isn’t horrified by what is going on. The internal tensions are building up – and we believe momentum is on our side.”

The Independent has asked the Conservative party for a response to Mr Allison’s comments and to explain why the group’s event has not been listed.