Britain's former top diplomat in Afghanistan has delivered a scathing attack on the conduct of UK military operations in the country. Sir Sherard Cowper-Coles accused the army of deploying "spurious" arguments and revealed a cabinet minister confessed to not knowing the difference between a Tornado jet and a torpedo.

The war in Afghanistan gave the army a raison d'être it had lacked for years, and "resources on an unprecedented scale", he said.

He added that, at one time, nearly 30% of all British helicopter movements in southern Afghanistan were for VIPs, mostly "senior military tourists from London".

The criticism appears in supplementary written evidence by Cowper-Coles to the Commons foreign affairs committee, released today. Cowper-Coles, who left the Foreign Office last autumn, was the ambassador in Kabul before being appointed as the special envoy for Afghanistan and Pakistan.

He appeared before the MPs in November. In today's evidence, he went further in his criticism, questioning the army's strategy when it deployed thousands of troops to Helmand.

He wrote: "In the eyes of the army, Afghanistan has also given our forces the chance to redeem themselves ... in the wake of negative perceptions, whether or not they were justified, of the British army's performance in Basra."

The then head of the army, General Sir Richard Dannatt, had told him in 2007 that, if he did not use the battle groups coming free from Iraq in Afghanistan, he would lose them in a future defence review.

Strategy in Helmand was thus driven as much by the level of resources available to the army as by "an objective assessment of the needs of a proper counter-insurgency campaign in the province".

What Cowper-Coles calls the "supply-side strategy" was reflected in the policy of rotating entire brigades through Helmand every six months. He said ministers were reluctant to question military advice for fear of leaks to the press suggesting they were not supportive of the troops.

Cowper-Coles continued: "I remember the RAF producing a paper arguing for Tornado bombers to be sent to Afghanistan, even when Nato ... made clear that the one category of weapons system [it] did not need more of was ground-attack jets."

The draft paper prepared for ministers argued that the aircraft were necessary for the morale of British forces on the ground – "not an argument that carried much weight with anyone familiar with the average British squaddie's view of the RAF," Cowper-Coles wrote. When he told a minister he might ask if it made sense to spend £70m on extra taxiways at Kandahar for Tornados, the minister replied that he could not do so as he "didn't know the difference between a Tornado and a torpedo".

The military blamed ministers unfairly for shortages of equipment in Helmand, Cowper-Coles wrote, adding: "I cannot help remembering an RAF movements officer in Helmand showing me a pie chart of British helicopter movements in southern Afghanistan ... 27% of the helicopter movements were for moving VIPs around theatre.

"And most of those VIPs were senior military tourists from London."