BRITAIN is to form a Vietnam-style ''air cavalry'' brigade equipped with gunship and troop-carrying helicopters to add punch and mobility to future peacekeeping and United Nations intervention operations worldwide, writes Ian Bruce, Defence Correspondent.

The bulk of the new unit will be drawn from two of the current three battalions of the Parachute Brigade, which has not jumped into action since the Suez invasion in 1956 and was in danger of having its manpower slashed under Treasury pressure for cost-savings on the annual #22bn defence budget.

The ''air cav'' option will be announced next month when details of the Government's year-long Strategic Defence Review are made public after final scrutiny and approval by the Cabinet.

One Para battalion will continue to train for parachute operations while the 1200 men of the other two learn to carry out rapid assaults from helicopters supported by the 67 US-designed and state-of-the-art Apache gunships now on order for the British Army.

The Apache, literally a flying tank, carries a devastating combination of 30mm cannon, anti-tank missiles and anti-personnel rocket pods. During the Gulf War, US Apache squadrons destroyed hundreds of Iraqi tanks and other armoured vehicles.

One crew even became flying shepherds in the final hours of the ground campaign when hundreds of Iraqi infantry emerged from bunkers to surrender to them. The captives were ordered to throw away their weapons and were then herded back towards Allied lines while the Apache hovered above them.

The new brigade will not be welcomed by all, but it reflects the reality that dropping men by parachute against even Third World opponents has become a suicidal mission with the spread of shoulder-fired anti-aircraft missiles.

Other changes expected in the review are a cut in the number of Challenger 2 main battle tanks in service, the scrapping of up to six surface warships to ease pressure on the naval budget for construction of two new large aircraft carriers, the slashing of Territorial force numbers from 59,000 to around 32,000, a partial withdrawal from Germany, and the provision of at least six heavy-lift transport aircraft.

Up to one-third of the 386 Challengers just beginning to be delivered could be mothballed on a care and maintenance basis or sold to Saudi Arabia as an interim measure.