The refusal of Mr Hayward, who announced his resignation on Tuesday, to travel to Washington was attacked by senior senators who said the company was already on "thin ice with the American people".

Senators are to continue pushing for Mr Hayward to appear, saying they wanted to question him over whether BP "advocated trading blood for oil".

The firm is also refusing to allow Sir Mark Allen- a former MI6 official who helped negotiate a valuable Libyan oil contract for BP with Colonel Gaddafi – to appear at the hearing.

BP has admitted that Sir Mark, an adviser to the firm, also spoke to Jack Straw, the former Justice Secretary, about Britain introducing a prisoner transfer agreement with Libya.

BP announced that it would instead send Peter Mather, the company's head of UK operations to appear before the Senate hearing. It is not clear whether this will be acceptable to the senators.

Robert Menéndez, a Democratic senator from New Jersey, said: "I would have thought that a company on thin ice with the American people for devastating the Gulf Coast would want to fully co-operate with our effort to fully understand the release of a terrorist who murdered 189 Americans".

Mr Menéndez, who will lead the Senate Foreign Relations Committee hearing into the issue on Thursday, accused BP and Hayward of being consumed by his "multi-million dollar golden parachute" as he steps down as the company's chief executive.

The senator vowed to keep pressing for Hayward to testify on "serious lingering questions about whether the company advocated trading blood for oil" and pressed for the bomber's release to safeguard a lucrative deal with Libya.

The hearing has been called to examine BP's alleged role in Scotland's decision last August to release Abdelbaset al-Megrahi, the only man convicted over the 1988 bombing of a Pan-Am flight that killed 270 people.

The Scottish authorities freed al-Megrahi on compassionate grounds after being assured he suffered from terminal cancer and had three months to live – but nearly a year later, he is alive in his native Libya.

The Scottish Government has denied that BP played any role in the release of al-Megrahi.

Scottish ministers and advisers have refused invitations to appear before the same Senate hearing. Mr Straw also rejected an invitation.