Margaret Thatcher wanted to threaten Saddam Hussein with chemical weapons after the Iraqi leader invaded Kuwait in the 1990 Gulf war, according to a top secret memo that has been declassified.

The high-risk gambit that could have led to Britain’s first recorded use of such weapons since 1919 underlines how hawkish Thatcher became in her last days as prime minister.

The documents show that Thatcher was countered by Dick Cheney, then US defence secretary, an unlikely dove.

Three years later, with Thatcher having been forced out as prime minister in November 1990, the UK signed the Chemical Weapons Convention, outlawing the production and use of such weapons.

Thatcher spent her last months in power desperately rallying western and Arab countries against the leader of Iraq, which invaded Kuwait in August 1990, while dealing with domestic problems from the poll tax to rises in tube fares.

A British armoured division prepares in Saudi Arabia for the Gulf war

Determined to defend Kuwaiti sovereignty, protect Saudi Arabia’s oil wells and destroy Iraq’s military machine, she told then President George Bush senior in August 1990 that it was “no time to go wobbly”.

Chemical weapons use was one of several areas where she disagreed with the US president. The CIA concluded that Saddam’s forces would use chemical weapons if they were about to be forced out of Kuwait, and that there would be “little warning time”.

Thatcher told Mr Cheney in October 1990 that “we had to decide what our response would be. If we wished to deter a CW attack by threatening to retaliate in like manner, we must have CW weapons [sic] available,” according to a minute of their meeting, classified top secret.

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Mr Cheney replied that Mr Bush had a “particular aversion to chemical weapons”. Although no final decision had been taken, US commanders were inclined “to rely on massive conventional response to a CW attack”, partly because they had “no experience” of using chemical weapons.

The exchange is omitted from Thatcher’s memoirs, which otherwise do not shy away from recording her aggressive stance. After the Gulf war ended in 1991, she did publicly suggest that she would have been prepared to use nuclear weapons against Iraq. But the declassified documents provide evidence that claim was a bluff.

Mr Cheney asked if she “could contemplate the use of nuclear weapons in a Gulf conflict. The prime minister said she would be most reluctant to consider this, indeed she would rule it out, although nuclear weapons were always there as the ultimate deterrent.” Mr Bush himself had been concerned about UK newspaper stories talking up the prospect of a nuclear retaliation against Saddam.

Thatcher’s conduct invites comparison with that of Tony Blair before the 2003 Iraq invasion.

Unlike Mr Blair, Thatcher tried to convince the US not to seek a further resolution from the UN Security Council, saying that it would put Iraq “on notice” of a coming invasion.

She was dismissive of the legalities: a November 1990 briefing said that there was little legal basis for the UK to destroy the entire Iraqi war machine, as she wanted. “But Saddam has said he will never leave Kuwait,” she wrote in the margin.

Thatcher also kept most of her cabinet in the dark about preparations for the conflict — writing on another document: “The fewer the people who know, the better. We have bad experience of secret papers leaking. MT”.

Emboldened by her experience in the Falklands, Thatcher saw the “whole reputation of the US and the UK” at stake in the Gulf.

In her last cabinet meeting, she increased the British contribution to the war effort to 45,000 personnel. In her memoirs, she wrote that not being in office to see the war through was “one of her very few abiding regrets”. She called the failure to disarm Saddam “a mistake which stemmed from the excessive emphasis placed right from the start on international consensus”.

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