Gordon Brown has ruled out holding a public inquiry into the Lockerbie disaster. In a letter to representatives of victims’ families, the prime minister said it would be “inappropriate” for the UK government to hold such an investigation.

His decision was criticised by relatives who said it has left them with little hope of learning the truth about who was behind the attack and the government’s handling of case.

UK ministers say it is now up to the Scottish government to decide if it wants to hold its own, more limited, inquiry into the worst terrorist attack on British soil. The SNP government has ruled out an independent inquiry, saying Holyrood lacks the constitutional power to examine the international dimensions of the case.

Supporters of a UK-led inquiry into the 1988 attack that killed 270 people believe it could involve sensitive Foreign Office files explaining why ministers named Libya as the state which sponsored the attack.

In a letter to Jim Swire, whose daughter Flora, 24, died in the attack, Brown said: “I understand your desire to understand the events surrounding the bombing of Pan Am flight 103.” But he added: “I do not think it would be appropriate for the UK government to open an inquiry of this sort.”

Brown rejected Swire’s claim that a break-in at Heathrow airport shortly before the plane carrying the bomb took off rendered the conviction of Abdelbaset Ali al-Megrahi unsafe.

“I understand the Court of Criminal Appeal in Scotland did consider whether the incident at Heathrow made the conviction of Mr Megrahi unsafe, and concluded that it did not,” the prime minister added.

Jean Berkley, whose son Alistair, 29, was killed in the bombing, said Brown had let down relatives of the victims.

“By handing it back to Scotland it looks like they are trying to push it into the long grass,” she said.

Professor Robert Black, a QC campaigning for an inquiry, accused Brown of “gross political cowardice”. He suspected the government did not want further scrutiny of sensitive aspects of the case which led the Scottish Criminal Cases Review Commission to report in 2007 that the conviction against Megrahi may be unsafe.

Black pointed out the Inquiries Act 2005 allows the UK government to hold a joint inquiry with Holyrood on matters straddling both parliaments. “The security of the United Kingdom and foreign relations are non-devolved issues and if Gordon Brown is saying these are not areas that would be involved in a Lockerbie inquiry then the man is insane,” he said.

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