Britain will be left “isolated” internationally if it leaves the European Convention on Human Rights (ECHR), the head of the body that oversees the agreement has said.

Alain Berset, the secretary general of the Council of Europe, said he was “absolutely ready” to discuss reforms to the ECHR amid growing criticism from governments that it was too outdated and inflexible to cope with the migrant crisis.

However, speaking to the Tories and Reform UK – both parties are committed to quitting the convention if elected – he claimed the move would weaken Britain’s influence internationally. 

Sir Keir Starmer, the Prime Minister, has consistently said the UK would not quit the ECHR under Labour.

Critics of the ECHR claim the advantage of leaving for the UK would be to take back control over human rights law, but Mr Berset told the BBC: “The opposite is true. What I see is more the risk to be a bit isolated. It would mean to be not participating to all the discussion on migration, to take an influence.”

It follows multiple cases, revealed by The Telegraph, of illegal migrants using the ECHR to avoid deportation, including an Albanian criminal who was allowed to stay in Britain partly because his son would not eat chicken nuggets.

The council, which is unrelated to the European Union, is the international political body in Strasbourg that oversees and enforces the work of the European Court of Human Rights.

The court rules on how to interpret human rights law in its 46 member states. In the UK, the government and judges must take these rulings into account, but are not bound by judgments that do not closely relate to our circumstances.

Sir Keir Starmer’s Government has pledged to tighten the way courts can interpret articles three and eight of the ECHR to prevent illegal immigrants and foreign criminals using the convention to frustrate their deportation from the UK. The two articles protect against persecution and rights to a family life.

Shabana Mahmood, the Home Secretary, has also called for reform of the convention, aligning the UK with nine EU nations, led by Denmark and Italy, which are also demanding major changes.

Such reform would need the agreement of all member states including the UK – which, under Sir Winston Churchill, was one of the key architects of the ECHR in the aftermath of the Second World War. This year marks its 75th anniversary.

In an interview with the BBC, Mr Berset acknowledged that human rights laws may need to “change or adapt” and that the institutions remained crucial to peace, security and justice.

“I am ready, absolutely ready, and really open to engage in all political discussions, to see what we need to discuss, maybe to change or to adapt,” he said.

“Let us engage on migration issues and to see what we need to address and maybe to change. The most important point is to be ready to speak on all issues without taboo … and to see then what could be the possible consensus between member states.”

While he said he would not comment on internal politics in the UK, Mr Berset appealed for the debate over the ECHR to return to “facts”.

Lord Blunkett and Jack Straw, both former Labour home secretaries, have called for the Government to consider suspending the ECHR or decoupling UK human rights laws from it, in order to enable ministers to deport more illegal migrants.

In September, however, Ms Mahmood warned that withdrawing from the ECHR would put Britain in a “club” with two other countries – Russia and Belarus – undermining the UK’s ability to secure agreements to combat illegal migration.