An RAF Tornado and its crew are missing after being downed by a US Patriot missile close to the Kuwaiti border.
A Tornado GR4 was brought down
The GR4, which is thought to have had a pilot and navigator on board, was from RAF Marham in Norfolk.
Air Marshal Brian Burridge, commander of British troops in the Gulf, said the Tornado had been hit "obviously by mistake" as it returned from a mission over Iraq.
William Farish, the US ambassador to Britain, told the BBC the loss of the plane was "absolutely terrible".
It comes after two helicopter crashes within the last three days led to the deaths of 19 coalition troops, including 14 Britons.
Three ITV news crew are also missing and feared dead, after coming under what their injured colleague claims was "friendly fire".
After the RAF plane went missing, Group Captain Al Lockwood, chief spokesman for the British forces, said: "This is a tragedy and we are taking rapid steps to find out the reason and to ensure that there is no repetition."
Air Marshall Burridge told the BBC's Breakfast with Frost programme: "This is a sad moment but we will put it behind us as quickly as we can in a military sense and carry on to our objective."
RAF Group Captain Jon Fynes said: "A situation like this does do not mean
anything of harm to the coalition but in many ways it brings us closer
"At the base they are sad it happened, but they are now refocused on what
needs to be done next."
In a statement Downing Street said the prime minister understands it is "an extremely difficult time for those involved, their families and colleagues of the crew who are missing".
Tory leader Iain Duncan Smith said: "I am afraid that it is the nature of warfare and you try to minimise this, but it is a tragedy nonetheless for the families."
The loss of the Tornado came after two Royal Navy Sea King helicopters from the Ark Royal collided over the Gulf on Saturday, with the loss of all seven crew members - six British and one American.
A memorial service has been held on board the aircraft carrier for those killed. So far, only one body has been recovered.
On Friday an American CH-46 Sea Knight crashed over the Kuwaiti desert killing eight British and four American service personnel.
The number of accidental deaths amounted to a difficult start to the campaign "and not an ideal one for the UK", Captain Lockwood said.
Coalition leaders are anxious to limit the chances of further accidental deaths and have taken measures against the dangers of friendly fire.
UK Defence Secretary Geoff Hoon, who sent his sympathies to the families of those missing, said there were a set of procedures in place to defend British and coalition forces.
"Sadly on this occasion they have not worked and we are conducting urgent reviews, both of the operation of missile batteries as well as of our own aircraft to ensure that this cannot happen again," he told the BBC's Politics Show.
US Marines' air controllers have been attached to UK forces to co-ordinate strikes against Iraqi positions.
vehicles have also been fitted with identification panels and
beacons, which can be switched on if they fear they are at risk from attack.
US-led forces have been encountering pockets of stubborn resistance as they press ahead towards the Iraqi capital Baghdad.
In one of the longest-running challenges so far in the conflict, air strikes were called in on the southern port town of Umm Qasr to overcome about 120 Iraqis.
They were thought to include elite Republican Guards
US aircraft have also bombed Iraqi positions in Nasiriya, where an estimated 500 Iraqis have confronted Marines, injuring or wounding about 50.