Gerry Adams’ convictions for attempting to escape from prison in the 1970s have been overturned by the UK’s highest court after it ruled that his detention was unlawful.

The former Sinn Fein leader took legal action claiming the two 1975 findings of guilt were unsafe because his incarceration was not “personally considered” by a senior government minister.

Mr Adams, 71, attempted to flee the Maze high-security jail near Belfast – also known as Long Kesh internment camp – on Christmas Eve 1973 and again the following July. He was later sentenced to a total of four-and-a-half years.

He said: “Today is about the judgment from the heart of Britain’s own legal system, from their Supreme Court, saying that they acted unlawfully.”

Internment without trial was introduced as Northern Ireland descended into sustained inter-communal violence in the early 1970s.

It is believed that nearly 2,000 men and women were imprisoned during its four-and-a-half years of operation, Mr Adams said.

People were arrested suspected of paramilitary involvement but much of the intelligence was flawed, the historic record suggests.

At a Supreme Court hearing in November, Mr Adams’ lawyers argued that, because the interim custody order (ICO) used to initially detain him in July 1973 was not authorised by the then-secretary of state for Northern Ireland Willie Whitelaw, his detention was unlawful and his convictions should be overturned.

Announcing the Supreme Court’s written judgment at a remote hearing on Wednesday, Lord Kerr – the former Lord Chief Justice of Northern Ireland – said the court had unanimously allowed Mr Adams’ appeal and had quashed his convictions.

The judge said Mr Adams’ detention was unlawful because it had not been “considered personally” by Mr Whitelaw.

Lord Kerr said: “The making of the ICO in respect of the appellant was invalid since the secretary of state had not himself considered it.

“In consequence, Mr Adams’ detention was unlawful, hence his convictions of attempting to escape from lawful custody were, likewise, unlawful.”

Lord Kerr added: “The appeal is therefore allowed and his convictions are quashed.”

The judge explained that Mr Adams had been detained under an ICO made under the Detention of Terrorists (Northern Ireland) Order 1972 and that “such an order could be made where the secretary of state considered that an individual was involved in terrorism”.

Lord Kerr said the power to make such an order was “a momentous one”, describing it as “a power to detain without trial and potentially for a limitless period”.

He added: “This provides an insight into Parliament’s intention and that the intention was that such a crucial decision should be made by the secretary of state.”

Mr Adams has consistently denied being a member of the IRA.

On Wednesday he said: “There is an onus on the British Government to identify and inform other internees whose internment may also have been unlawful.”

“I consider my time in the prison ship Maidstone, in Belfast prison and in Long Kesh to have been in the company of many remarkable, resilient and inspiring people.”

He went on to lead Sinn Fein for decades before retiring as president in 2018 and did not contest his seat in the Irish parliament during this year’s general election.

He added: “Our journey into the cages of Long Kesh and the other places came through torture centres, came through special courts, came through no courts at all.

“Remember what the British did – they set aside due process, they brought in special coercive legislation and what the Supreme Court judgment does today is to set that clearly in that context and to say it was unlawful.”

A series of other legal cases could be affected by the decision, lawyers said.

Ulster Unionist leader Steve Aiken said IRA victims will be “angered and bewildered”.

He added: “Mr Adams is able to go into a British court more than 40 years later and seek legal redress before getting off on a technicality.

“That is a lot more than hundreds of victims of the IRA are able to do.”