Leading lawyers and legal experts are warning that Brexit could trigger a human rights crisis in the UK that threatens to have a ‘domino effect’ across Europe.

In a letter to the Observer, they claim that the UK’s departure from the EU may see the removal of fundamental rights from UK law, leaving its citizens with a similar level of legal protection to people in Belarus, a dictatorship.

Signed by more than 50 prominent lawyers and academics, including Helena Kennedy, Sir Geoffrey Bindman, Professor Francesca Klug, Lord Lester and Sir Paul Jenkins, the letter states: “Theresa May has repeatedly stated her feelings that Britain would be better served by leaving the European Convention on Human Rights (ECHR) than it would leaving the EU. As we enter Brexit negotiations, there is now every possibility that both these scenarios could easily come to pass.”

It continues: “The ECHR has been the bedrock of peace in Europe since the second world war, and was instrumental in the remarkable growth of democracy in Eastern Europe after the fall of the Berlin Wall. It is no coincidence that the one state that is not part of the Convention, Belarus, is known as “Europe’s last dictatorship”.”

The letter will put pressure on the prime minister to set out how the UK intends to respect the rule of law and maintain limits on the state’s powers post-Brexit.

To join the European Union, countries need to be a signatory to the European Convention on Human Rights which was incorporated into EU law through the Lisbon Treaty. But Brexit raises questions about what legal guarantees will be given to its citizens once the UK has left the EU.

The letter’s signatories claim: “We face the very real threat of a human rights crisis with the UK trading away protections against torture for grubby trade deals with foreign tyrants. We are calling for the EU to make Britain’s membership of the ECHR a legally binding requirement for any future free trade deal with the UK. The rule of law and human rights are non-negotiable when new countries join the EU, they should be non-negotiable when countries leave and desire a free trade deal.

The letter’s publication comes as campaigns agency 89up launches a crowd fundraiser to take a battle bus carrying human rights activists and campaigners to Brussels to lobby the EU to ensure any free trade deal that it agrees with the UK will include a guarantee that the UK remains a signatory to the European Convention on Human Rights.

“We’re raising funds to take a bus full of campaigners to Brussels, so the EU can hear from British people that we want our human rights guaranteed in any free trade deal,” said Sashy Nathan, 89up’s director of advocacy. “Theresa May’s plans to leave both the EU and the ECHR could trigger a domino effect of leaders across Europe abandoning human rights.”