By David Fairhall, Defence Correspondent

Britain has agreed to buy the American Trident missile as part of a £5 billion programme to replace the Royal NavyÕs Polaris submarine force in the 1990s.
The terms of the deal, embodied in an exchange of letters between Mrs Thatcher and President Carter, are almost identical to those of the Nassau Agreement of 1962, which preceded the Polaris sales agreement.

The effect will be to maintain the same type of nuclear deterrent through into the second decade of the twenty-first century, but with the ability to launch nearly three times as many warheads against Soviet cities from each submerged submarine, and to launch them from a wider area.

The Trident missile, already in service with the US Navy, has the additional ability to direct each of its eight warheads to a separate target Ñ the so-called MIRV system.

Under the new Anglo-American deal the missiles, their fire control system and MIRV device, will be bought off the shelf from US manufacturers, leaving the actual nuclear bombs to be installed by British engineers at Aldermaston.

Four, and possibly five, nuclear-powered submarines will be built in British shipyards to take sixteen missiles each, together with new base facilities at Coulport and Faslane, in Scotland.

The American missile package will cost about £1 billion, including a 5 per cent contribution to research and development. The total cost of a four-ship Trident force is calculated at £5 billion Ñ plus another £600 million if a fifth ship is considered necessary Ñ of which an estimated 70 per cent will be spent in Britain over the next fifteen years. 

Mrs Thatcher has promised to assign the new force to NATO Òexcept where supreme national interests are at stake.Ó 

The GovernmentÕs long-predicted decision comes at a time when BritainÕs need for an independent nuclear deterrent is vigorously debated within the Labour Party and when public concern about nuclear issues is probably greater than at any time since the early 1960s.

Within the defence establishment the choice of Trident is also intensely controversial. Despite Ministry of Defence claims that it will absorb only 5 per cent of the military equipment budget over the next 15 years, and the financial room made for it by this weekÕs decision to abandon the MBT 80 tank project in favour of Challenger, fears remain that BritainÕs conventional military contribution to NATO in the late 1980s will be weakened.

The Government admits that these alternatives could be a lot cheaper but has rejected them as not being cost-effective by comparison with TridentÕs assured ability to incinerate the Soviet leaders in their Kremlin offices.