More Tornados from RAF Marham are set to join the Nato-led air operation over Libya, Prime Minister David Cameron said last night - as the head of the service warned it was stretched to the limit.

Tornados have been operating from the Gioia del Colle base, in southern Italy, since they joined the UN-backed operation in Libya two weeks ago.

Marham aircraft have carried out armed reconnaissance missions, destroying tanks, armoured vehicles and ammunition dumps.

During a visit to the Italian base yesterday, Mr Cameron said four more GR4s from Marham-based IX(B) Sqn would be deployed within days.

The latest deployment will increase the number of UK fast jets taking part in the international effort to protect Libyan civilians from dictator Muammar Gaddafi to 22 aircraft.

The announcement came as Mr Cameron arrived on a surprise visit to the Italian airbase, which has been the temporary home for the last two weeks for 10 Typhoon fighters and eight Tornados, six from Marham and two from RAF Lossiemouth in Scotland.

Mr Cameron said the British people should be 'proud' of the efforts of the RAF pilots and crew, who he said had saved the lives of 'literally thousands' of Libyans by holding back Gaddafi's forces from opposition strongholds such as Benghazi.

The RAF jets have flown more than 70 sorties from southern Italy, with Typhoons from XI(F) Squadron of RAF Coningsby in Lincolnshire patrolling the Libyan no-fly zone, while the Tornados target the tanks and armour of Gaddafi's ground forces.

Aircraft have also taken off from Marham to attack targets in Libya, using air-to-air refuelling to make the 3,000-mile round trip from Norfolk.

Over the weekend, RAF warplanes destroyed three of Col Gaddafi's tanks and 10 armoured vehicles, the Prime Minister said.

Mr Cameron said he wanted to say 'thank you and well done and a heart-felt thanks from the British public to the brave Typhoon and Tornado pilots and their crews, who have done an incredible job in a short period of time to save literally thousands of lives in Benghazi and Libya'.

He added: 'The whole country should be proud of what they have done.

'They have responded incredibly quickly. They have flown many sorties. They have been extremely successful in holding back Gaddafi's forces.

'Just over the weekend, they have destroyed 10 armoured vehicles and three tanks and they have flown a huge number of missions very rapidly and, as ever, very brilliantly.

'I want to say a big thank you to them and also make an announcement that four new additional Tornados will be deploying in the next couple of days, which will mean we will have 10 Typhoons for the mission in terms of the no-fly zone and a total of 12 Tornado ground attack aircraft involved in operations.'

The arrival of the Tornados will help ease pressure on the international coalition from the US decision to withdraw its planes from frontline duties from last Saturday, having provided the bulk of the firepower in the first weeks of operations.

Mr Cameron's visit came on the day when the head of the RAF, Air Chief Marshal Sir Stephen Dalton, warned that the service would need 'genuine increases' in its budget in the years to come if it is to continue running the range of operations demanded by ministers.

And it came as ministers gave details to the House of Commons of Army and Royal Navy redundancies forced by the 8pc reduction in defence budgets imposed in last year's spending review.

Sir Stephen told The Guardian that the RAF was planning for operations over Libya to continue for at least six months.

While he accepted the imperative for reform, he said the RAF would need an 'uplift' in spending well before 2020 and claimed that Mr Cameron had acknowledged this requirement.

'The key factor is that if we are to meet the requirements laid upon us, there is no question that more investment will be needed to achieve that,' the Air Chief Marshal said.