Alleged British use of chemical weapons in Mesopotamia in 1920

It has been alleged that the British used chemical weapons in Mesopotamia in 1920, during the Iraqi revolt ("Ath Thawra al Iraqiyya al Kubra"), in the period of the colonial British Mandate. Use of tear gas and lethal poison gas against rebels was considered, and the use of tear gas was promoted by Winston Churchill, head of the War Office, as he argued it would frighten Iraqi rebels and cause them to flee while not actually killing anyone nor leaving serious or lasting effects on those caught in the tear gas and that the use of non-lethal tear gas was an overall better solution than the bombing campaign which failed to quell resistance to British rule in Iraq. Practical difficulties prevented its use rather than any moral inhibitions.
Historiography.
Historian Charles Townshend made the “first assertion of British chemical weapons use in Iraq” in his 1986 essay "Civilisation and "Frightfulness": Air Control in the Middle East Between the Wars":
Britain was not a free agent in the middle east, and would have to defer to the universal prejudice against all forms of gas. In vain did the Air Ministry stress that lethal concentrations were most unlikely to be reached under air bombardment (because - though this point was not stressed - of its low accuracy). In vain did they point out that the army had used SK gas shells in quantity against the Mesopotamian rebels in 1920 with 'excellent moral effect'.
In his book "World Orders Old and New", Noam Chomsky claimed that Churchill was particularly keen on chemical weapons, suggesting they be used "against recalcitrant Arabs as an experiment". Churchill dismissed objections to the use of chemical weapons as "unreasonable" and stated: "I am strongly in favour of using poisoned gas against uncivilised tribes". 
In March and July 1992, US Representative Henry B. González, speaking in the House of Representatives, claimed that Britain used gas against the Kurds.
Criticism of allegations.
The main source usually quoted in support of the idea that Britain used poison gas in Mesopotamia is Geoff Simons, "Iraq: From Sumer to Saddam" (1994), who stated that "gas was used against the Iraqi rebels in 1920". In the third edition of his book, "Iraq: From Sumer to post-Saddam" (2004), Simons wrote: "In the event, gas was used against the Iraqi rebels in 1920 with 'excellent moral effect', though gas shells were not dropped from aircraft because of practical difficulties".
Another historian, Lawrence James, stated, "By September the local commander, General Sir Aylmer Haldane, was beginning to get the upper hand, although he was still desperate enough to clamour for large supplies of poison gas. It was not needed, for air power had given his forces the edge whenever the going got tough". On whether gas was used he writes that: "RAF Officers asked Churchill... for use of poison gas. He agreed but it was not used".
Niall Ferguson, in his 2006 book "The War of the World", wrote: "To end the Iraqi Insurgency of 1920... the British relied on a combination of aerial bombardment and punitive village burning expeditions. Indeed, they even contemplated using mustard gas too, though supplies proved unavailable". Anthony Clayton wrote in "The Oxford History of the British Empire" that "the use of poisonous gas was never sanctioned".
A December 2009 article in the "Journal of Modern History" by R.M. Douglas of Colgate University went through the known sources and concluded that "[
while at various moments tear gas munitions were available in Mesopotamia, circumstances seeming to call for their use existed, and official sanction to employ them had been received, at no time during the period of the mandate did all three of these conditions apply" and that it was clear that no poison gas was used. Douglas observed that Churchill's forceful statement had served to convince observers of the existence of weapons of mass destruction which were not actually there, which ironically matched events in 2003.
British policy.
The British 1914 "Manual of Military Law" stated that the rules of war applied only to conflict "between civilized nations". It clearly stated that "they do not apply in wars with uncivilized States and tribes", but the British commander should observe "the rules of justice and humanity", according to his own individual discretion.
Some gas shells and protective clothing were shipped to India in July 1919, with a further small shipment in January 1920, for use on the North-West Frontier. However, a requisition for 16,000 shells and 10,000 gas masks was blocked by Lord Sinha, the Under-Secretary of State for India. He believed that a first use of chemical weapons by British and Indian forces would have serious implications, both moral and political, and that chemical weapons should be used only in retaliation for an Afghan or North-West Frontier Tribal chemical attack. In India, a temporary Travelling Gas School was set up in September 1920, but the matter then lapsed.
Britain had used gas weapons in the Middle East, most notably in the Second Battle of Gaza against Ottoman forces in World War I. On that occasion, the use of gas did not prevent a British military defeat.
The use of tear gas, not "only the most deadly gases", was considered; as shown in a War Office minute of 12 May 1919 in which Winston Churchill argued:
 "I do not understand this squeamishness about the use of gas. We have definitely adopted the position at the Peace Conference of arguing in favour of the retention of gas as a permanent method of warfare. It is sheer affectation to lacerate a man with the poisonous fragment of a bursting shell and to boggle at making his eyes water by means of lachrymatory gas. I am strongly in favour of using poisoned gas against uncivilised tribes. The moral effect should be so good that the loss of life should be reduced to a minimum. It is not necessary to use only the most deadly gasses: gasses can be used which cause great inconvenience and would spread a lively terror and yet would leave no serious permanent effects on most of those affected."