Prime Minister David Cameron on Saturday expressed Britain's "unconditional admiration" for the families of the victims of the Lockerbie bombing on the attack's 25th anniversary.

Pan Am Flight 103 blew up over the Scottish town of Lockerbie on December 21, 1988, killing all 259 people on board and another 11 people on the ground.

Libyan Abdelbaset Ali Mohmet al-Megrahi was the only person convicted of the attack, but he maintained his innocence until his death last year.

Cameron said Saturday's landmark should instead focus on the victims "and to those whose lives have been touched and changed by what happened at Lockerbie that night".

"Let us say to them: 'our admiration for you is unconditional'," he said in a statement.

"For the fortitude and resilience you have shown. For your determination never to give up. You have shown that terrorist acts cannot crush the human spirit. That is why terrorism will never prevail.

"Even in the darkest moments of grief, it is possible to glimpse the flickering flame of hope."

Cameron highlighted the actions of Syracuse University, which lost 35 of its members in the attack.

The university honours the dead each year with the Remembrance/Lockerbie scholarships, which are presented to two Lockerbie students and 35 of its own undergraduates.

"This is the lasting and optimistic legacy bequeathed to future generations on behalf of those who lost their lives on this day 25 years ago and who we remember here today," he added.