Abd al-Basit al-Migrahi is serving a life sentence in a Scottish prison after being found guilty of murdering 270 people with a bomb that blew up a Pan Am airliner over the Scottish town of Lockerbie just before Christmas 15 years ago.
Three judges from the original trial - which took place at a special court in the Netherlands - gave the minimum term ruling at the High Court in Glasgow, a court official said.
Lord Sutherland, one of the judges, said in the ruling: "Quite clearly this was a wicked act carried out in the full knowledge that the plan, if successful, would result in slaughter of many entirely innocent persons."
Al-Migrahi was originally sentenced to life in January 2001 with a recommendation that he should serve 20 years.
But under human rights laws introduced in late 2001, criminals sentenced for life must be told exactly how many years they have to serve before they can apply for parole.
Victims' relatives unhappy
Al-Migrahi's lawyer, Eddie MacKechnie, said outside the court: "As far as the future is concerned, all this has done is accent the need for us to marshal our forces to establish the innocence of this man."
"I have no doubt we will have the opportunity for a fresh appeal in due course, though it may be months away," he told Sky news.
But relatives of the victims said the minimum term was not long enough.
Kathleen and Jack Flynn lost their son John Patrick on the flight. He would have celebrated his 36th birthday on Monday.
"Twenty-seven years is one month per victim. I mean really, when you look at it, that's how short it is," Jack Flynn said.
His wife said: "We're not vindictive people, we're not looking for vengeance - we're looking for justice and justice is not 27 years."
The court official told Reuters that the judges, Lords Sutherland, McLean and Coulsfield, said the prison term would be backdated to April 1999.