Britain's involvement in the Libya conflict will cost the taxpayer as much as £1bn if it continues into the autumn as expected, according to expert analysis and data gathered by the Guardian.

Two months after western powers began bombing Libyan targets to protect civilians in Operation Unified Protector, the cost to Britain so far of the dozens of bombs dropped, hundreds of sorties flown and more than 1,000 service personnel deployed is estimated at more than £100m, according to British defence officials.

But defence economists have told the Guardian the costings are conservative. Francis Tusa, editor of the Defence Analysis newsletter, estimates that by the end of April Libyan operations had already cost the UK about £300m and that the bill was increasing by up to £38m a week.

Nato action in Libya: click image for the graphic and analysis

Military chiefs have acknowledged that the air campaign would last six months. At this rate, the Ministry of Defence's own estimates will put the cost of war at about £400m, but the expert view is that the figure will top £1bn by September.

Another defence analyst told the Guardian £1bn was probably at the top end of the scale, but that it would not be a complete surprise in Whitehall if this was the final bill for six months of operations.

"A lot of what they are doing out there is a substitute for training that would have cost anyway," he said.

"The final cost will depend on whether the Treasury is prepared to pay for replacements for all the bombs and missiles that have been used so far."

British warplanes are increasingly involved along with the French and Italians. According to data collected by the Guardian for the six weeks of aerial operations up to 5 May, the British have flown 25% of nearly 6,000 sorties over Libyan skies – second only to the Americans. The US total was inflated by an early surge, and it has now scaled back its operations. For the five weeks to 5 May , Britain flew more sorties than any other country. But British planes have been dropping far fewer bombs than their allies, relative to the number of flights .

So far, they have attacked about 300 targets, perhaps only 10% of the few thousands destroyed by Nato aircraft.

Interactive guide to Nato attacks in Libya. Click image for graphic

Norway and Denmark have by some distance the highest ratio of bombs dropped in relation to population.

The true cost of the operation will not be announced for weeks, according to defence officials. It is certain to be significantly more than the "tens of millions" stated in parliament by the chancellor, George Osborne, shortly after the bombing started. One other thing is certain: the cost of the bombs has been significantly more than the targets they have destroyed.

The Nato operation was designed to implement a UN security council resolution authorising force to defend civilians. But after stopping Muammar Gaddafi's forces wresting back chunks of the east of the country, the campaign has had little discernible impact in recent weeks on Gaddafi's stronghold in the capital.

Tripoli has been heavily bombed for the past 10 days, with all Libyan fighting ships either sunk or damaged and many command posts and bunker complexes also hit. However, demonstrations in support of Gaddafi are still common and dissident groups are unwilling to engage his loyalist army, which still controls the west of the country. Defence chiefs in the UK and US are also said to be concerned that some Nato countries are unwilling to commit air power to the campaign.It is not only the cost that is worrying the Ministry of Defence, and, indeed, defence chiefs in the Pentagon.The reluctance of most countries to commit their air forces to action – Norway, which has dropped about 300 bombs, is to pull out at the end of June.– is causing serious concern among military commanders throughout the alliance about whether Nato countries have the political will and military capability to continue operations that now have the stated aim of removing power from Gaddafi, his sons, and closet advisers.

For Britain, the Libyan conflict has also presented military commanders and ministers alike with an uncomfortable reminder of the perilous state of the defence budget. As Paul Cornish, head of the international security programme at the thinktank Chatham House, has observed, many of the military capabilities used in and around Libya – HMS Cumberland, the Nimrod R1 eavesdropping plane, the Sentinental surveillance aircraft, and Tornado jets – are among the first casualties to be scrapped or their numbers reduced (in the case of Tornados) as a result of last year's strategic defence and security review.

"The obvious question to ask," Cornish writes in the latest issue of The World Today, "is whether Britain could have made a contribution to the intervention in Libya had the crisis developed later in 2011 when most of the decommissionings, disbandments, and retirements would otherwise have taken place."

The US led the assault, during the first week flying more than 800 sorties in Libya, of which over 300 were strike sorties. It fired more than 200 Tomahawk cruise missiles from its ships. Britain has fired fewer than 20 Tomahawks, costing an estimated £1m each, from the submarine HMS Triumph.

Britain, which has accounted for some 25% of all sorties, was so worried about the gap left by the US when it ceded command to Nato, and stood down its aircraft – including low-flying A10 tankbusting "Warthogs" and C130 gunships