Sarah Bryant was the first female British soldier killed in Afghanistan
The UK's operation in Afghanistan is "worthless" and akin to the start of the Vietnam war, former SAS commander Maj Sebastian Morley has said.
Speaking to the Daily Telegraph, he said the government had "blood on its hands" over the "unnecessary" deaths of four soldiers.
BBC defence correspondent Caroline Wyatt said many on the ground felt the campaign has been "under-resourced".
But the MoD insisted the security challenge was "manageable".
The former SAS commander resigned after Cpl Sarah Bryant and three of her colleagues died when their Snatch Land Rover hit an anti-tank mine in Helmand province in June 2008.
Cpl Bryant was the first female soldier to die in Afghanistan.
Maj Morley, 40, said he was compelled to stand down after Quentin Davies, the Minister for Defence Equipment and Support, told an "unacceptable lie" in the wake of the deaths.
Mr Davies had said commanders had a choice of vehicles, although he has subsequently said he had not meant to cause any offence.
Speaking for the first time since his resignation, Maj Morley launched a scathing attack on the state of the military campaign as a whole.
"This is the equivalent to the start of the Vietnam conflict, there is much more to come.
"We hold tiny areas of ground in Helmand and we are kidding ourselves if we think our influence goes beyond 500 metres of our security bases.
"We go out on operations, have a punch-up with the Taleban and then go back to camp for tea. We are not holding the ground."
And, addressing the use of Snatch Land Rovers, which he deemed to be unsafe and prompted his decision to stand down, he said: "I had to resign.
"I had warned (the MoD) time and time again that there were going to be needless deaths if we were not given the right equipment, and they ignored this advice. There is blood on their hands.
"There was no other vehicle to use. The simple truth is that the protection on these vehicles is inadequate and this led to the unnecessary deaths."
The BBC's Caroline Wyatt said the ex-commander sounded "profoundly disillusioned" by his experiences", adding that there were an "incredibly diverse" range of views on the progress of the campaign in Afghanistan.
However, she said: "It is a campaign that many people on the ground say has been under resourced [and] under funded on all sides."
Our correspondent pointed to the US' decision to deploy a further 17,000 military personnel to Afghanistan as evidence that the current situation in the country is problematic and there is a desire for a new strategy.
A spokesman for the MoD acknowledged that the Snatch was not suitable for high-risk environments, but was "mission critical" for certain operations.
He added that new technology and state-of-the art armoured vehicles were continuing to reach Afghanistan, and the anticipated arrival of additional US troops meant the fight would be taken to the Taleban with greater vigour than ever.
"It is true that in an area the size of Helmand there is a limit to how much ground we can hold," the spokesman said.
"But that does not mean we are not making progress. We are.
"The security challenge is manageable by the available forces and the overriding mood of the local population is one of optimism and hope."
He added: "Quentin Davies has already made clear that any offence caused by remarks he made on the issue was entirely inadvertent."