The former Sinn Fein leader says his detention on the orders of a junior minister was invalid.

Lawyers for Sinn Fein’s former leader Gerry Adams appealed to the United Kingdom’s highest court on Tuesday to try to overturn two convictions from the darkest days of the violence in Northern Ireland in the 1970s.

Adams was held without trial under special “internment” measures designed to quell increasing unrest over British rule of the province in the early days of “The Troubles”.

He tried to escape in December 1973 and July 1974 from the high-security Maze prison near Belfast, which housed pro-British loyalist and republican paramilitaries alike.

But the escape plans, reportedly orchestrated by the Irish Republican Army (IRA), both failed and he was convicted of both attempts in March and April 1975.

Adams, now 71, did not challenge the guilty verdicts at the time but is now attempting to have them overturned following the release of previously classified government documents.

He argued his detention on an interim custody order under anti-terrorism legislation was invalid because it was not considered by the secretary of state for Northern Ireland.

Court of Appeal judges in Belfast dismissed the case in February 2018, saying they were satisfied the order was valid as the junior minister who signed it was acting on behalf of his superior.

But the Supreme Court gave permission for a further appeal and Adams’s lawyer Sean Doran told five judges on Tuesday the order was “unlawful” and his client’s convictions were unsafe.

He claimed British courts and the government later accepted there was “no room for subordinate officials to play a role” in decisions to detain suspects.

“We say the only appropriate person would be a secretary of state,” he told the court, adding: “The appellant’s case was not in fact considered by the secretary of state.”

The panel of five Supreme Court judges will deliver their verdict at an unspecified later date.

Internment

Internment was a controversial policy when it was introduced to try to end the spiralling violence between pro-British loyalists and republicans who wanted a united Ireland.

Between 1971 and 1975 nearly 2,000 mainly republican prisoners were held without trial, according to Ulster University.

The policy – considered by some as a curb on civil liberties – is often credited for stoking the bloodshed and bolstering support for the IRA.

After his release from prison, Adams became a political figurehead during some of the bloodiest episodes of the conflict, which left 3,500 dead over three decades.

He was charged with IRA membership in 1978 but the case was dropped because of insufficient evidence.

He became leader of Sinn Fein – which has long been associated with the IRA – in 1983.

But he has always publicly denied being enrolled in the ranks of paramilitary leadership, despite repeated conflicting claims from movement insiders.

Adams was elected to the British parliament but declined to take up his seat under Sinn Fein’s policy of abstention, in which representatives refused to swear loyalty to the British head of state.

The IRA – which did not recognise the authority of the British state to imprison its members – frequently carried out prison breaks.

The most prolific saw 38 republican paramilitaries escape from the Maze prison in 1983, in a huge symbolic victory for the group.

The Troubles effectively wound down with the 1998 Good Friday Agreement, which ushered in a power-sharing government in Belfast between republican and unionist parties.

Adams stood down as Sinn Fein leader in February 2018 but is still considered a leading force of the movement to create an all-Ireland republic.