The shadow defence secretary, Clive Lewis, has revealed he nearly shot a civilian while serving as a soldier in Afghanistan.

The former territorial army officer, who suffered depression after returning from his three-month tour, admitted the incident while criticising law firms for hounding soldiers over their roles in Iraq.

The Labour MP for Norwich South said that if troops were left uncertain about what support they would have from the armed forces over “an accident”, it could impact on the effectiveness of combat.

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Speaking at a fringe event at the Labour party conference in Liverpool, he said: “The key thing is our soldiers should be able to deploy and know that they have the support to be able to make decisions in the best interest of their forces on the ground.

“I can speak from knowledge here, from personal experience. When I was in Afghanistan there were situations – and I can think of a situation in particular – where I could have easily shot a civilian. It could have happened. Very close.

“And I needed to know – I was given the reassurance that as long as I had followed certain rules and procedures, I knew that I would be protected by the state, by the armed forces. I wouldn’t be hounded for what was in effect an accident under very strenuous circumstances.

“If you begin to throw doubt on that and you make people begin to think twice, then you basically in effect degrade combat effectiveness and put people in danger. There is a balancing act between being able to operate freely and effectively but also to ensure that civilians and others are protected in the appropriate way.”

Lewis said it was right that soldiers alleged to have broken international humanitarian law should be investigated, but left alone if cleared. The MP, who was elected last year, was an army reservist inventory officer in 7th Battalion the Rifles between 2006 and 2009, serving in Afghanistan in his final year.

At the Royal British Legion event, about the challenges facing armed services personnel as they make the transition into civilian work, he spoke of his own difficulties, including battling depression. The former journalist also said he was left distressed when a newsroom colleague asked: “So how many people did you kill?”