Adam Busby, the self-declared leader of the hardline Scottish National Liberation Army, has avoided prosecution for alleged terrorism offences after a court ruled he was medically unfit to stand trial.

Busby, 66, was due to be tried for multiple hoax bomb threats in the UK after being extradited from Dublin this year. He had allegedly threatened to bomb Scottish bridges and to poison water supplies in England.

The SNLA, a tiny but long-lived republican splinter group, has been implicated in hoax and attempted letter bomb attacks on members of the royal family, including the Prince of Wales; nearly every prime minister since the 1970s, including Margaret Thatcher, Gordon Brown and Tony Blair; as well as the former Scottish first minister Jack McConnell. No one was injured.

The Crown Office has disclosed that Busby was discharged by a court in Glasgow on Wednesday after medical experts unanimously declared he was too mentally and physically infirm to stand trial. Busby has been diagnosed with multiple sclerosis, a condition that led to delays in initial extradition attempts.

Busby has been on remand in prison since his extradition. Police and prosecutors are expected to monitor his behaviour to ensure he does not start reoffending, the Crown Office indicated on Thursday.

“Based on unanimous expert medical opinion, Adam Busby has been assessed as unfit to stand trial at this time,” a spokesman for the Crown Office said. “Should there be a change in that situation, the crown reserves the right to re-raise proceedings.”

He has previously been named by prosecutors in the US as the architect of a concerted campaign of hoax bomb threats and blackmail attempts against the University of Pittsburgh.

The US has not yet made a formal application for him to be extradited. His poor medical health and release by the Scottish courts makes it unlikely that an application for him to be rearrested and extradited to the US would succeed.

The self-proclaimed founder of the SNLA, Busby fled from Scotland to the Republic of Ireland in 1980 after facing prosecution over alleged terrorist offences in the UK following a spate of incidents by radical, hardline nationalists, including attempted letter bomb attacks.

He allegedly used his base in Dublin to organise further hoaxes involving anthrax weapons and fake bombs, and real attacks including incendiary devices in parcels, by influencing other hardline nationalists in Scotland and England.

In 2010 he was sentenced to four years in jail by a court in Dublin for sending email threats from a public library to BAA at Heathrow, claiming bombs were on two flights to New York. He was convicted of similar offences in 1997 by an Irish special criminal court.

Scottish police and prosecutors have sought to have him extradited ever since he fled Scotland.

He also faced extradition to the US to face trial for 35 charges of fraud, international extortion and more than 40 hoax bomb threats against the University of Pittsburgh in 2012. A grand jury indictment said those threats cost the university more than $300,000 (£183,000) in extra security and baggage checks.

The latest trial involved threats that explosives were placed at places such as the Forth road bridge near Edinburgh, the Erskine bridge over the Clyde west of Glasgow, the Argyll shopping arcade and the Hilton hotel in Glasgow.

The indictment said Busby claimed in those calls that “a bomb or other thing” was liable to explode or ignite, and contacted the Samaritans in Glasgow with the same claim.

In 2009, Busby was also accused of contacting a newspaper threatening to “contaminate the drinking water” of English towns and cities. He is alleged to have claimed that packages containing “a noxious substance” were sent to people not named in the charges.