British military personnel serving in Libya will receive an operational allowance in recognition of the "rigour and risk" they face, Defence Secretary Liam Fox said on Wednesday.

UK servicemen and women operating in Libyan airspace and territorial waters will be paid an extra £29.02 a day tax-free, backdated to the start of military action in March.

The allowance, which is already paid to British troops serving in Afghanistan, doubled in May from £14.51 per day, fulfilling a pledge by the coalition government.

Announcing the move, Fox said he wanted to reward those service personnel "engaged in the very highest areas of conflict".

"Eligibility for the operational allowance reflects the rigour and risk of the operation in question and I am delighted that we can now extend these payments and recognise more of our brave servicemen and women in this way," he added.

The allowance for troops involved in the Nato mission to enforce a no-fly zone over Libya will be paid out of the Treasury Reserve.

Pilots and air crews carrying out missions over Libya and the crews of ships and submarines operating within 12 nautical miles of the coast will receive the payment for each day they serve there.

Speaking at the Royal United Service Institute in London, Fox stressed that the UK was committed to pursuing operations in Libya for "as long as it takes".

"No-one can predict how long a complex intervention will take -- every scenario will be different, militarily, politically and diplomatically," he said.

"Sustaining the tempo does increase the pressure on both personnel and equipment as planning assumptions are tested, and it tests the ability of defence companies to support frontline operations."

International forces, including Britain, launched air strikes on Libya in March under a UN resolution aimed at protecting civilians from attacks by troops loyal to the country's veteran leader Moamer Kadhafi.