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US Senate hearings on whether BP oil deals influenced the release of the Lockerbie bomber have been postponed because key witnesses declined to take part.

Senator Robert Menendez accused witnesses, including outgoing BP boss Tony Hayward, of "stonewalling".

Former UK Justice Secretary Jack Straw and Scottish Justice Secretary Kenny MacAskill have also refused to attend.

A British diplomatic source denied the UK government had stonewalled.

The source, in Washington, said the government had provided a "clear, comprehensive and credible account" of its understanding of the circumstances that lead to Megrahi's release, and Prime Minister David Cameron had met with four US senators "at very short notice" last week.

"It would have been extremely unusual for a parliamentarian of one country to be held accountable to the parliamentarians of another," he said.

"But more importantly, individual testimonies from former British ministers would not have materially added to anyone's understanding of what, how and why Megrahi was released by the Scottish Executive."

'Diplomatic tennis'

US politicians are looking into claims the release of Abdelbaset al-Megrahi was linked to oil deals with Libya.

The hearing, due to be chaired by Senator Menendez of New Jersey, was due to get under way on Thursday.

Announcing the postponement - which he stressed was a not a cancellation - the senator said it was "outrageous" none of the key witnesses would co-operate.

"We don't intend for this to be forgotten or swept under the rug," he said.

"They have stonewalled. Each side has claimed innocence, each side has blamed the other.

"It is a game of diplomatic tennis that is worthy of Wimbledon, but not worthy on behalf of the lives of the families who still have to deal with this terrorist act and the consequences of the loss of loved ones in their lives."

"In the case of BP, it is hard to imagine that a company on such thin ice with the American people, after devastating our Gulf Coast, would not fully co-operate in getting to the bottom of the release of a terrorist who murdered 189 Americans," he added.

It has been suggested BP wanted Megrahi released to curry favour with Libya.

The Scottish government, which released Megrahi last year on compassionate grounds after doctors said he was dying from prostate cancer, has denied it had any contact with BP before deciding to free him.

Libyan Megrahi is the only man to have been convicted of the bombing of Pan Am flight 103 on 21 December 1988.

The flight, from London to New York, exploded 31,000ft over Lockerbie, in south-west Scotland, 38 minutes after take-off from London, killing all 259 people on board, along with 11 people on the ground.

Megrahi was released from a Scottish prison in August 2009 having been given three months to live but is still alive almost a year later.