The attacks were intensified from May, six months before the United Nations resolution that Tony Blair and Lord Goldsmith, the attorney-general, argued gave the coalition the legal basis for war. By the end of August the raids had become a full air offensive.

The details follow the leak to The Sunday Times of minutes of a key meeting in July 2002 at which Blair and his war cabinet discussed how to make “regime change” in Iraq legal.

Geoff Hoon, then defence secretary, told the meeting that “the US had already begun ‘spikes of activity’ to put pressure on the regime”.

The new information, obtained by the Liberal Democrats, shows that the allies dropped twice as many bombs on Iraq in the second half of 2002 as they did during the whole of 2001, and that the RAF increased their attacks even more quickly than the Americans did.

During 2000, RAF aircraft patrolling the southern no-fly zone over Iraq dropped 20.5 tons of bombs from a total of 155 tons dropped by the coalition, a mere 13%. During 2001 that figure rose slightly to 25 tons out of 107, or 23%.

However, between May 2002 and the second week in November, when the UN Security Council passed resolution 1441, which Goldsmith said made the war legal, British aircraft dropped 46 tons of bombs a month out of a total of 126.1 tons, or 36%.

By October, with the UN vote still two weeks away, RAF aircraft were dropping 64% of bombs falling on the southern no-fly zone.

Tommy Franks, the allied commander, has since admitted this operation was designed to “degrade” Iraqi air defences in the same way as the air attacks that began the 1991 Gulf war.

It was not until November 8 that the UN security council passed resolution 1441, which threatened Iraq with “serious consequences” for failing to co-operate with the weapons inspectors.

The briefing paper prepared for the July meeting — the same document that revealed the prime minister’s agreement during a summit with President George W Bush in April 2002 to back military action to bring about regime change — laid out the American war plans.

They opted on August 5 for a “hybrid plan” in which a continuous air offensive and special forces operations would begin while the main ground force built up in Kuwait ready for a full-scale invasion.

The Ministry of Defence figures, provided in response to a question from Sir Menzies Campbell, the Liberal Democrat foreign affairs spokesman, show that despite the lack of an Iraqi reaction, the air war began anyway in September with a 100-plane raid.

The systematic targeting of Iraqi air defences appears to contradict Foreign Office legal guidance appended to the leaked briefing paper which said that the allied aircraft were only “entitled to use force in self-defence where such a use of force is a necessary and proportionate response to actual or imminent attack from Iraqi ground systems”.

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