Tony Blair last night paid tribute to four soldiers killed in the Middle East as MPs again called into question Britain's military campaigns in Iraq and Afghanistan.

The three UK troops killed in Afghanistan included two from the Household Cavalry Regiment and one from 7 Parachute Regiment Royal Horse Artillery; they died when their patrol vehicle came under attack from insurgents in the Helmand province, the bloodiest day for British forces since deployment in the region.

Just four hours earlier, Corporal Matthew Cornish, 29, serving with the 1st Battalion Light Infantry in Iraq became the first to die in an attack on a UK military base there.

The fatalities pushed the number of British service personnel who have died in Iraq since the start of hostilities in 2003 to 115 and saw the number of soldiers killed in Afghanistan in the past two months climb to nine.

Last night Mr Blair redrafted a foreign policy speech in Los Angeles to speak of British troops' great "bravery and sacrifice".

"It brings home yet again the extraordinary courage and commitment of our armed forces who risk their lives and, in some cases, tragically lose them defending our country's security and that of the wider world," he said. "These are people of whom we should be very proud."

But politicians responded to the deaths by calling on the Government to clarify its military strategy.

Labour MP Alan Simpson, a long-term critic of the military campaigns, said the deaths were further evidence that Afghanistan had become "mission impossible", while Iraq was "mission irretrievable".

"The money the US threw at the warlords when they went in pursuit of Osama bin Laden now leaves the British troops trying to deal with an enemy that is just as independent but much better armed than they have been before," he said. Liberal Democrat shadow defence secretary Nick Harvey said the deaths in Afghanistan under-lined the need for a clear military strategy.

"This is a vital mission but, with the head of Nato forces describing the country as 'close to anarchy', the Government must be clear about the challenges ahead," he claimed.

In one of the deadliest strikes against British troops stationed in Afghanistan in recent months, Taliban militants armed with rocket-propelled grenades and heavy machine guns ambushed the soldiers - part of the Nato-led international security assistance force - while on patrol. A fourth soldier was evacuated to a military hospital in a critical condition.

Responding to yesterday's heavy death toll, Defence Secretary Des Browne paid tribute to the efforts of British troops and stressed his commitment to operations in both countries.

"Those responsible for the attacks on our soldiers in Northern Helmand do not want to see security and prosperity brought to the local people," he said. "We cannot allow them to succeed."

The attack in Afghanistan comes a day after the British general leading the Nato troops assumed command of 8,000 multi-national forces in the lawless south of the country.

It is believed to be the first time since the Second World War that a UK general has commanded units of American troops in combat operations.