An historic agreement setting limits to the nuclear arsenals of the two super-Powers was signed by Mr Nixon and Mr Brezhnev last night - the most vital of the US-Soviet pacts that have flowed from the American President's visit to Moscow.

The pact on missiles is the first direct step taken by the two giants to slow down this side of the nuclear arms race after over a quarter of a century of confrontation in the cold war. There are two parts to it:

• A formal treaty placing limits on the number of anti-ballistic missiles (ABM) for defence, designed to ensure that neither side could be invulnerable to attack or be tempted to launch a surprise offensive.

• An executive agreement under which the two sides will place limits on the numbers of certain types of offensive land-based and submarine-based missiles. This accord is to be confirmed later by another formal treaty.

Agreement on the arms limitation came after a two and a half hour meeting between Brezhnev, the Soviet party leader, and Mr Nixon, in the Kremlin. It followed two years of negotiation by diplomats in Vienna and Helsinki.

The treaty, which has to be ratified in the US, will be of unlimited duration, but either country can terminate it on six months notice if it feels that to continue it would jeopardise its supreme interests.

Though the construction of submarine-launched ballistic missiles on all nuclear submarines will be frozen at the present levels, the agreement does not freeze the number of warheads which can be fitted to a missile. The development of American missiles with multiple warheads offsets the advantage the Russians have with the larger number of intercontinental missiles (ICBMs) under the freeze.

Under the agreement any further construction of submarine-launched ballistic missiles must be accompanied by the dismantling of an equal number of older, land-based ICBMs or older submarine launchers.

At a dinner preceding the signing, Mr Nixon said: "It is an enormously important agreement but, again, it is only an indication of what can happen in the future as we work towards peace in the world. I have great hopes on that score."

He hinted he would invite Soviet leaders to the US and "in some way respond in an effective manner to the way in which you have received us so generously in your country."

The Soviet Premier, Mr Kosygin, said that the agreement would "go down in history as a major achievement on the road towards curbing the arms race. This is a great victory for the Soviet and American peoples."

At the signing ceremony, there was genial hand-shaking all round, at times excitedly, one Soviet journalist declaring: "This is not only the first visit of an American President to Moscow - this is a turning point in the history of the world."

Mr Laird, the US Secretary of Defence, meanwhile was saying in Washington that the arms accord would slow down the development of Soviet strategic weapons but still require an acceleration of some US offensive weapons programmes. It should not lead to relaxation of the US guard.

He called for accelerating development of a new generation of nuclear missile-firing submarines known as the Trident, and going ahead with development of the air force's new strategic B-1 bomber.