Blow for Gordon Brown as defence aide quits over Afghanistan strategy

On the eve of a major speech defending the British campaign in Afghanistan, Gordon Brown suffered a serious blow when the parliamentary aide to defence secretary Bob Ainsworth quit, telling the prime minister that public support for the war will collapse "unless politicians are seen to treasure those who fight for us".

Eric Joyce, a former army major and Ainsworth's parliamentary private secretary, also condemned what he described as "petty, back-channel attacks" on the military by Labour politicians.

Joyce, MP for Falkirk, is known to be furious at what he regards as invidious attacks on the former head of the British army Sir Richard Dannatt by Labour defence ministers. They had claimed Dannatt was acting politically by demanding better equipment for British soldiers.

Joyce states in his resignation letter: "I do not think the public will accept for much longer that our losses can be justified by simply referring to the risk of greater terrorism on our streets."

And in an article for the Channel 4 News website, he writes: "Petty, back-channel attacks on respected senior officers are indistinguishable from attacks on the services as a whole."

He also calls for greater clarity in Britain's objectives in Afghanistan: "It should be possible soon to say that our commitment there will reduce markedly during the lifetime of the next government. The public wants to see much more coherence and direction in that respect."

Joyce claims his resignation was timed not to be disruptive.

A furious Downing Street argued Joyce was a maverick who had resigned before he had heard Brown's case. Sources pointed to pressure he faced over his expenses.

Joyce also complains that President Obama needs to show greater appreciation for the British effort. "For many, it seems that Britain fights; Germany pays; France calculates; Italy avoids. If the US is seen as valuing each of these approaches equally then I think they may well end up shouldering the burden themselves."

Ainsworth is understood to have tried to persuade Joyce from resigning, knowing the timing could not have been worse. In a statement Ainsworth stressed Joyce's junior role as an aide and said it was vital he had a ministerial team that was "fully committed to our mission in Afghanistan".

Downing Street released the text of a response from Brown replying to Joyce: "Dear Eric, Thank you for your letter. Bob Ainsworth has already replied to you emphasising the importance of the effort in Afghanistan to defeating terrorism, and of supporting, at all times, our armed forces. I am determined that nothing will distract my government from this most vital of tasks."

The resignation came as Brown prepared to mount a passionate defence of the British military campaign in Afghanistan tomorrow in a speech arguing that British troops will not leave until the Afghan government is able to look after its own security and fend off the Taliban. In his speech Brown argues: "People ask what success in Afghanistan would look like. The answer is that we will have succeeded when our troops are coming home because the Afghans are doing the job themselves."

The speech comes as the deaths were announced of two more British soldiers in southern Afghanistan – 52 have now been killed since the start of July. In the face of growing signs that the war is unpopular, Brown will insist that if Nato leaves, al-Qaida will return to the country and use it as a base to mount attacks on the west.

Flanked by Ainsworth, and the new head of the army, General Sir David Richards, Brown will use his speech to the Institute of Strategic Studies to give some sign of an eventual exit. He will say the aim of the campaign is to ensure that the Afghan army and police are able to maintain security, and once that is achieved, Nato will leave.