Gerry Adams is to consider a court appeal after failing in a legal bid to overturn two historic convictions.
Mr Adams said he was disappointed but not surprised by the Court of Appeal judgment in Belfast.
The former Sinn Fein president had challenged the legality of two convictions received in 1975 relating to escape bids while he was detained without trial at the Maze Prison during the early 1970s.
His lawyers argued that his imprisonment was unlawful because of flaws in the detention process.
They claimed the order used to intern him was not legal, so he therefore should not have been prosecuted for trying to escape from what they asserted was unlawful custody.
The lawyers said Mr Adams's internment was not lawful because the interim custody order to hold him had not been considered by the then Secretary of State, and was instead signed by a junior minister at the Northern Ireland Office.
They contested that the 1972 Detention of Terrorists Order required senior level authorisation.
Three Court of Appeal judges rejected that argument on Wednesday.
"The Court of Appeal was satisfied that the ICO (Interim Custody Order) was valid having been made by the Minister on behalf of the Secretary of State," read the judgment.
"The Court was accordingly satisfied that the appellant's convictions are safe and dismissed the appeal."
Reacting to the judgment, Mr Adams said: "I am disappointed, though not entirely surprised, by today's High Court decision."
He added: "I will now discuss with my legal advisers what options, including an appeal, are open to me."
The Louth TD was first detained in March 1972, but was released in June that year to take part in secret talks in London.
He was rearrested in July 1973 at a Belfast house and interned at the Maze Prison, also known as Long Kesh internment camp.
Mr Adams had challenged subsequent convictions handed down by two separate Diplock Court trials - cases tried by a single judge sitting without a jury - relating to two attempts to escape from internment.