TWO Royal Navy aircraft carriers, HMS Invincible and HMS Illustrious, will this week lead Britain’s Trafalgar celebrations.

The ships will take pride of place in the 100-strong fleet assembled in the Solent to mark Nelson’s victory over the combined French and Spanish fleets 200 years ago.

But the great admiral might turn in his grave were he to get wind of the plans quietly being laid for the ageing ships’ replacement.

The Royal Navy’s two new multi-billion-pound aircraft carriers, which will be the most powerful warships ever to fly the White Ensign and be the cornerstone of the future fleet, may be partly built in France. The drive towards a joint programme is likely to trigger considerable political controversy and at the same time determine the future shape of Britain’s shipbuilding industry.

The rapprochement with the old enemy is being driven by high politics and low pragmatism. British ministers are eager for a big Anglo-French defence project to show that Britain is not totally reliant on America for defence tchnology and that, despite recent travails, the entente cordiale is alive and well.

The Ministry of Defence, meanwhile, sees an opportunity to make its aircraft-carrier plans fit an overstretched budget.

The Royal Navy wants two new 65,000-tonne carriers, which most experts believe will cost £4 billion or more to build. But the budget available is only £3.5 billion, according to defence industry executives and Whitehall insiders.

Co-operation with the French, who need their own new carrier, holds out the prospect of significant savings for Britain, with development and construction costs spread across three ships rather than two.

The first hints of a link came in June last year. Geoff Hoon, then defence secretary, signed a co-operation agreement with his French counterpart Michele Alliot-Marie on the deck of the Charles de Gaulle when the French nuclear-powered carrier made a visit to Portsmouth.

But in recent weeks momentum for a joint-carrier programme has quickened.

The main French objection — that the British carrier design might not be suitable for Rafale, the fighter aircraft the French will base on the ship — appears to have been overcome, and defence companies from each side have begun talks on how industrial co-operation might work. The French must decide whether to join the UK programme by October.

Senior defence industry executives told The Sunday Times at the recent Paris air show that the way was now clear for a joint carrier programme.

“We have an agreement with the French. They will build one-third of the ships, and we will build two-thirds,” said the chief executive of one leading British defence group.

Others are cautious. “It is by no means a done deal. The balance between the risk and the benefits of co-operation does not make the decision blindingly obvious,” said one source.

It is understood that under the preliminary plans discussed between the two governments and shipyards on both sides of the Channel, the Royal Navy would receive the first new ship, the French the second and the British the third.

Plans for a joint programme may not be universally well received. Defence executives involved in the talks warn that co-operation with the French risks delays to the start of the programme, leaving British shipyards starved of work in the interim.

There are few big military programmes in the offing apart from the carriers. Some yards, in particular Swan Hunter, the Tyneside yard close to Tony Blair’s Sedgewick constituency, face an alarming drop in work if the carrier is delayed to accommodate the French.

Shipbuilding unions on both sides of the Channel may also oppose the move, arguing in each case that it could lead to a loss of work for their members.

The Royal Navy may be worried that its proposed in-service dates for its two ships, 2012 and 2015, would be jeopardised by a joint approach.

The carrier programme also has the potential to cut across a MoD-led initiative to weld Britain’s remaining naval shipyards — which are spread across the country and belong to several different owners — into a single, cohesive company.

Officials at the Defence Procurement Agency have been in talks with BAE Systems, owner of the naval yards on the Clyde and the submarine yards at Barrow-in-Furness, Babcock, which operates the naval base at Rosyth, VT Group, which has yards on the south coast, and DML, which runs the Devonport naval base and yard. But a source close to the talks said these carefully laid plans could be short-circuited by the allocation of work on the carriers.

“Once they decide which bits the French could do, and which bits will be allocated to which British yards, that will decide the future composition of the combined UK shipbuilding organisation. The carrier work is in reality the main asset that these yards will have,” said the source.

One executive involved in the negotiations said: “It is a positive development. Having three ships in the programme and spreading the work over a number of years is a sensible move, providing the yards with a steady workload, something they haven’t had for years.”