Britain is to spend £500 million on hundreds of heavily armoured vehicles to protect troops in Afghanistan.

The decision to acquire 600 vehicles that can withstand landmines and roadside bombs comes after criticism of the Ministry of Defence from the families of servicemen killed while on patrol in lightly armoured “Snatch” Land Rovers.

Susan Smith, from Tamworth in Staffordshire, whose 21-year-old son Private Phillip Hewett died in Iraq in 2005, is suing the MoD for providing vehicles that “gave little or no protection against improvised explosive devices”.

Des Browne, the Defence Secretary, has been negotiating with the Treasury over the plan to buy the 600 vehicles, which will partly replace the Land Rovers but also add to the stock of heavier troop-carrying systems in Iraq and Afghanistan. It is hoped that the new vehicles will be in place by next spring, in time for an expected resurgence of Taleban fighting.

The sense of urgency surrounding the provision of better protection for troops was underlined yesterday when it was confirmed that 100 of the 600 vehicles would replace the Viking armoured vehicles, which came into service with the Royal Marines in 2006 and have been deployed across Helmand province in southern Afghanistan. The Viking has proved to be vulnerable to landmines. About a dozen Viking drivers from the Royal Marines and the Queen’s Royal Lancers have been killed or seriously wounded by mine strikes, and steps were taken to thicken the armour under the driver’s seat.

Under the new proposal, the Vikings will be replaced by 100 better-protected “high-mobility tracked patrol vehicles”. The 500 other new vehicles will consist of 100 Mastiffs, a huge British-modified version of the American Cougar troop-carrier, 100 Jackals, a mine-resistant open-top vehicle, and 300 light-support mine-protected vehicles, which have not yet been identified.

General Lord Guthrie of Craigiebank, the former Chief of the Defence Staff and now a patron of the United Kingdom National Defence Association, which campaigns for better conditions for the Armed Forces, said: “Defence has been underfunded and risks have been taken with security and the size and equipping of our Armed Forces for many years. The Government has been forced by events and the conflicts in Iraq and Afghanistan to urgently find additional funding, but it is too little and too late. Despite repeated warnings our services are neglected and under-resourced. This has cost lives.”

When casualties occurred during attacks on Snatch Land Rovers in Iraq, the MoD began looking at other armoured vehicle options. However, in 2006, when British troops were deployed to Helmand province in Afghanistan, more than 100 Snatch Land Rovers went with them and, as in Iraq, proved vulnerable to explosive devices planted by the insurgents. Casualties continued to rise.

Corporal Sarah Bryant, 26, of the Intelligence Corps, and three SAS reservists, were killed on June 17 in a Land Rover hit by a landmine in Helmand. She was the first woman to die in action in Afghanistan.

A spokesman for the MoD said that Snatch Land Rovers would remain in Afghanistan and would still be used in certain circumstances. The Mastiffs, for example, are too big to manoeuvre down narrow streets. There are 85 Mastiffs already in Iraq and Afghanistan and 60 Jackals in Helmand province. An earlier order of vehicles — 175 Mastiffs, 70 Jackals and 150 Ridgbacks, a smaller version of the Mastiff — are on their way to Afghanistan.

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