Sir Bill Cash, the veteran Eurosceptic for whom Brexit has been a lifetime's work

Eurosceptics have long insisted that any Brexit deal must pass "the Bill Cash test" before they can back it – and with good reason.

Sir Bill, 80, has spent the greater part of his adult life campaigning for Britain to leave the European Union and is recognised as the leading authority in Parliament on European law. Conservative prime ministers have learned to dismiss him at their peril.

His refusal to accept Theresa May's version of a Brexit trade deal was a rallying point for backbench MPs, and he was one of the first in the party to push seriously for Boris Johnson to be Mrs May's replacement.

He was also a leading campaigner for a referendum on membership of the EU which eventually ended David Cameron's premiership after he granted, and then lost, the national poll.

Almost 30 years ago, Sir Bill was a leader of the Maastricht rebellion, encouraging Eurosceptic Tory MPs to vote against the Maastricht Treaty in a forerunner of the row that toppled Mrs May.

John Major – like Mrs May – had a slim majority, of just 18 in his case, leaving him vulnerable to repeated ambushes by the rebels, which almost brought down his Government.

In one famous incident, Sir Bill secretly arranged for a seriously ill Tory MP to be flown from Scotland to take part in the vote, hiding him from the whips until the moment of the vote, which the Government lost.

He then set up the European Foundation to fund legal challenges to the Government on European issues, and the in-fighting over Europe that continued for the following years contributed to Mr Major's downfall and Tony Blair's election victory in 1997.

One Prime Minister who needs no reminding of Sir Bill's influence is Mr Johnson.

Sir Bill's connections to the Johnson family date back to before the Prime Minister was born. A friend of Stanley Johnson during their time as students at Oxford University, he knew Boris Johnson as a child and told Margaret Thatcher that his dispatches from Brussels as The Telegraph's correspondent there provided the most accurate insight into what was going on.

Mr Johnson was later appointed as Sir Bill's literary executor and stayed at his Grade I listed Elizabethan manor house in Shropshire.

After he became Prime Minister, Mr Johnson sought Sir Bill's advice rather than shunning it as his predecessors had done.

Before last year's Conservative Party Conference, he invited him to Downing Street to discuss the problem of the Northern Ireland backstop he had inherited from Mrs May.

Sir Bill was instrumental in forming to solution to the backstop, later set out by Mr Johnson in a letter to Jean-Claude Juncker, the then president of the European Commission.

An MP since 1984, Sir Bill has been a member of Parliament's European Legislation select committee since 1985 and has been practising law since 1979.

A father of two, he has been married to Bridget since 1965. Their son William Cash is a journalist, who wrote last year about the discovery that the family were distant cousins of the American singer Johnny Cash.