Scotland's justice secretary has been urged to make a statement to parliament on a predecessor's claim that Scottish ministers sought "concessions" from the UK Government to expedite the transfer of the Lockerbie bomber to Libya.

Scottish Liberal Democrat leader Willie Rennie said Kenny MacAskill's revelation in his book The Lockerbie Bombing: The Search for Justice contradicts previous statements on Scottish ministers' involvement in Tony Blair's prisoner transfer agreement with Libya.

The "deal in the desert" with former Libyan leader Colonel Gaddafi would have seen Abdelbast al-Megrahi, the only man convicted of the bombing of Pan Am Flight 103 over Lockerbie in 1988, transferred from Greenock prison to serve the rest of his sentence in his home country.

In the book, Mr MacAskill said he told Westminster officials the deal would cause "political difficulties" for the Scottish Government.

He said: "I explained that this would be made easier if they were able to offer some concessions to assist us ... the request for (concessions) was simply an opportunity to try to gain some benefits for Scotland from decisions that were clearly going to be taken anyway."

Al-Megrahi was subsequently released by Mr MacAskill on "compassionate grounds" and the former MSP was later replaced as justice secretary by Michael Matheson.

First Minister Nicola Sturgeon has yet to reveal her cabinet for the SNP's next term in government following the Scottish election but Mr Rennie said her new justice secretary's first job must be a statement on the claims in Mr MacAskill's book.

Mr Rennie said: "In both 2007 and 2009 the First Minister said he had no involvement in the prisoner transfer agreement and had not been consulted.

"Now, Kenny MacAskill is claiming that Scottish ministers were actively involved and were trading away their objections in return for more devolved powers.

"People will be concerned. Many were alarmed at the time that Tony Blair struck a deal in the desert with Colonel Gaddafi. Now it is suggested that Scottish ministers were involved at the highest level in attempts to trade off their personal concerns in return for new Scottish powers.

"The first item of business on the new cabinet secretary for justice's desk must be coming to parliament to set the record straight.

"We need to know whether the discussions that Mr MacAskill refers to in his book took place, who else was involved in these talks and what SNP ministers thought was a fair price for their silence."

A total of 270 people died when Pan-Am flight 103 blew up above Lockerbie on December 21 1988.

An SNP spokesman said: "Megrahi's release was on medical grounds alone, and political, economic and diplomatic considerations played no part in the SNP Government's decision.

"The same cannot be said of the UK Labour Government, which cosied up to Gaddafi and his regime when Jack Straw was in charge of the Foreign Office - a process which saw Tony Blair conclude the now infamous 'deal in the desert'."

A Scottish Government spokesman said: "As we know, al-Megrahi had a terminal illness and was released on compassionate grounds alone, based on the rules and regulations of Scots law and the reports of the Parole Board for Scotland, the prison governor and the Scottish Prison Service director of health and care.

"We expect the First Minister to make an announcement regarding the appointment of cabinet secretaries and ministers later this week."