Another grim milestone for armed forces as death toll in Afghanistan hits 300

The sombre milestone of the 300th fatality among British forces in Afghanistan has been reached with the death of a Royal Marine from injuries received in Helmand. He had been injured in a blast in the Sangin district on 12 June.

The announcement of the death was accompanied by reports that the UK's special envoy to Afghanistan and Pakistan, Sir Sherard Cowper-Coles, has gone on "extended leave" after expressing scepticism over Western military strategy and calling for speeding up talks with the Taliban.

Sir Sherard's tenure has become increasingly difficult after he clashed with US and Nato officials, and the Afghan President, Hamid Karzai, was said to have been furious after hearing that the ambassador had allegedly called for his removal. He is being temporarily replaced by Karen Pierce, the Foreign Office's South Asia director.

The departure of the veteran diplomat will strengthen opposition to the war, which has grown as the human and financial cost has increased. Yesterday demonstrators outside Downing Street demanded the immediate withdrawal of British forces.

The latest serviceman to die, from 40 Commando, was receiving treatment at Birmingham's New Queen Elizabeth Hospital. The Prime Minister, who had warned the British people to expect more casualties this summer, which is likely to see some of the fiercest fighting since the start of the war, said: "It's a moment for the whole country to reflect."

David Cameron continued: "We are paying a high price for keeping our country safe, for making our world a safer place and we should keep asking why we are there and how long we must be there. We are there because the Afghans are not yet ready to keep their own country safe and to keep terrorists and terrorist training camps out of their country."

Critics are questioning whether waiting for Afghanistan to become safe can remain a feasible aim as the scale of the violence escalates.

A Nato helicopter was brought down yesterday, the second in the last few weeks, leaving eight dead. Sixty Western soldiers have been killed this month, making June the deadliest month for Nato troops since the US-led invasion of 2001. British losses for the year stand at 76.

Major Renny Bulmer, of 40 Commando, said of the fallen Marine: "His courage and sacrifice will not be forgotten." He added that the thoughts of his comrades were with his family, who had kept vigil at his bedside.

The 100th and 200th deaths in the conflict also led to concern about the war when they happened. Yesterday Paul Gamble, the father of Private Daniel Gamble, who died in 2008 and was the 100th casualty, told ITV News: "The fact that Daniel was killed, he was the 100th, but his death is no more or less significant than any of the others... Every number is too large isn't it?

"The families are always going to be suffering but they're out there to do a job and until that job is done then there are going to be casualties."

A UN Security Council report issued at the weekend showed that roadside bombings had risen by 94 per cent in a year and, on average, there were three suicide bombings a week. Assassinations of Afghan government officials and local leaders who opposed the Taliban have risen by 45 per cent. Major-General Gordon Messenger, the British military's spokesman on Afghanistan, said: "I think it is right that people are questioning why we are in Afghanistan. I would think it wrong if we were not, given the sacrifices that the UK and many other nations are making. We are clear about what we are trying to do there and we are making progress."

He said roadside bombs had become a part of daily life for the troops. "The threat is getting more lethal in that the IEDs are widespread."