John Hutton said the reinforcements should have been dispatched in the spring – before the beginning of the attack on Taliban strongholds in Helmand and when he was still in government.
He spoke out as the Prime Minister announced a decision "in principle" to increase the number of troops in Afghanistan from 9,000 to 9,500.
Mr Brown could not say when the extra troops would be deployed as he set out a series of conditions, including guarantees that they will have proper equipment, before the extra 500 were sent to Afghanistan.
Mr Hutton, who stood down from the Cabinet in June, criticised the Government's delay in reinforcing the UK presence. "I think it would have been much more helpful had we had the additional troops there six months ago," he told BBC2's Daily Politics.
Last spring, military chiefs won the support of Mr Hutton when they asked Downing Street to send 2,000 more troops to Afghanistan. But Mr Brown, under pressure from the Treasury, only agreed instead to the temporary deployment of 700 troops to provide security for the Afghan elections in August. It soon became permanent at the request of British commanders.
The Prime Minister's veto is believed to have been one of the factors behind Mr Hutton's resignation.
Mr Hutton warned yesterday of the dangers of trying to cut corners in Afghanistan. He said: "If you do an economy-of-force operation here I think you could screw it up really badly."
He also hit back at calls for British troops to be pulled out of the warzone: "It would be a catastrophe, not just for our national security, but for the cohesion of the North Atlantic Alliance."
Last night, Ministry of Defence sources dismissed his comments on the timing of the deployment, insisting there were not enough trained troops available in the spring to send to Afghanistan.
Mr Brown's promised deployment follows a torrid summer for UK forces and signs of growing public opposition to the war. He listed the 37 British servicemen who have been killed since the last session of Prime Minister's Question Time 13 weeks ago, but argued: "When the safety of our country is at stake, we can't walk away."
Mr Brown told the Commons that the extra 500 troops would be dispatched when three conditions were met: that they are adequately equipped; that other Nato countries bear more of the strain in Afghanistan; and that more Afghan troops are sent to Helmand.
Last night, government sources were unable to specify when the deployment could take effect, beyond saying they hoped it would happen "as soon as possible".
The MoD has acknowledged there are serious difficulties in supplying British forces with military equipment. It has said there would not be enough armoured vehicles for 9,000 troops until next year, and equipping the extra 500 could stretch the time lag to spring 2011.
The Government has spent around £1bn on armoured vehicles for Helmand in the last few years. But some of them have failed to provide adequate protection against roadside bombs and are already having to be replaced.
Ministers are also finding it difficult to point to Nato countries – apart from the United States, where President Barack Obama is considering sending up to 40,000 more US troops – which are prepared to bolster Western forces in Afghanistan.
The Prime Minister said his proposed moves followed "clear military advice from the Chiefs of Staff and our commanders on the ground" and would help reduce the risk to British forces. He said 500 troops already in Afghanistan would be be switched from Kandahar to Helmand immediately.
He also announced that the Afghans would set up a military headquarters in Helmand, and British forces would "partner" 5,000 of the 10,000 Afghan soldiers being training in the province.
The head of the Army, General Sir David Richards, who has long been calling for troop increases, said: "Having sufficient troops on the ground is the key to this campaign; the additional numbers will enable us to maintain momentum in Helmand.
"I'm confident the Prime Minister's conditions can be met, and we will now get on with the military planning to ensure that soldiers in Afghanistan have the appropriate equipment for the tasks we ask them to undertake."
In their first Commons clashes since July, David Cameron told Mr Brown that cash cuts meant officers in the Territorial Army could miss out on the training they needed before being sent to Afghanistan.
Citing the case of an unnamed serving officer, he said: "Volunteers being asked possibly to lay down their life in the service of their country are not getting the basic training they need. This is totally unacceptable." But Mr Brown insisted TA training for Afghanistan was "properly resourced and will continue to be properly resourced".
* The White House was yesterday forced to deny claims made on the BBC's Newsnight that President Obama had agreed to send up to 45,000 troops to the country as early as next week. Mr Obama is currently locked in talks with his national security team over the future of the US mission and is considering whether to follow the advice of General Stanley McChrystal, the US and Nato commander in Afghanistan, that a major troop surge is necessary. Senior figures within his administration, including Vice-President Joe Biden, instead want to scale back the US presence and focus on targeting al-Qa'ida.
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