The new Afghan mission may not stretch the armed forces beyond breaking point but it will leave them "near the edge", a senior defence official admitted Tuesday.

Ministry of Defence insiders rejected claims that the deployment would leave them overstretched but service chiefs are certain to use the growing strains from the many overseas commitments to press claims for extra resources in this summer's public spending round.

With the decision to send more troops to Afghanistan, 31,000 out of 205,000 uniformed personnel will be on operations overseas or in Northern Ireland.

The "overstretch" arises because at any given time, the same number may be recovering from operations or preparing to replace those deployed. Gaps between postings get shorter. Although young people may welcome repeated operations, they have less time to be trained for them. Personnel and their families are put under additional stress, with the result that too many people leave.

Service chiefs have not disguised their concern that they will be unable to maintain the fast tempo of overseas deployments without more resources.

Whether they win the latest round of the long-running battle with the Treasury will be decided this summer in the spending review. The Ministry of Defence is hoping for a £1bn boost on top of inflation to its annual £24bn budget.

The decision to send 1,700 Royal Marines and commando troops to Afghanistan comes just as the Ministry of Defence is going through a long consultation to determine what capabilities it needs for the war against terrorism following the September 11 attacks on the US.

Geoff Hoon, defence secretary, has said there may need to be more rapid reaction troops, such as the Royal Marines and the Parachute Regiment. This represents a considerable step-up since the 1998 strategic defence review, on which the present forces' structure is based. Then, it was believed deployments would not be beyond the Gulf region.

The Afghan deployment is timely for the spending round because it is the kind of operation that Mr Hoon identified for the so-called "new chapter" of the defence review - for which he will want extra funding.

However, officials and defence experts played down the effect the Royal Marine deployment would have. They are kept ready for assignments such as this, sometimes spending up to six months afloat, waiting for operations or exercises. They do not take part in other routine rotations of British troops, so there is no strain on other operations. They can be relieved by fellow Royal Marines, or troops from other countries.

Officials also said marines had been on standby to go to Afghanistan for five months, either at sea in the Indian Ocean or at their UK bases. "We planned for this," an official said.

Meanwhile, the UK contribution to the international security force in Kabul, now 1,800 troops, will fall sharply in the coming months in keeping with the government's aim to be at the cutting edge when operations begin but later to pass responsibility to other countries. Germany has taken control at the brigadier level in Kabul, and the government is hoping that Turkey will take over leadership in April or May.

But the difficulty will come if a large part of the British contribution to the Afghan war, now about 6,000, has to be repeatedly replaced - or if a new crisis develops.