Two SAS members were killed when their RAF Puma helicopter crashed in Iraq, sources said.
Another two British military personnel were seriously injured when the aircraft came down near Baghdad on Tuesday night.
The Ministry of Defence (MoD) said it was too early to speculate on what caused the crash, but the US military revealed it is not believed to have been caused by enemy fire.
Regular British forces are based in Basra, southern Iraq, and UK units operating elsewhere in the country are often involved in special forces missions.
The MoD refused to comment on the service or regiment of those killed, but sources indicated they belonged to the SAS. An MoD spokesman said: "We do not comment officially on SAS matters. We are not going to confirm whether these people were members of the SAS."
The injured personnel are now recovering in hospital, and the next of kin of those involved have been informed.
An MoD spokesman said: "It is too early to speculate on the cause of the crash. An RAF board of inquiry has already been convened and is en route to Iraq to conduct its investigations. Our thoughts are with the families at this very difficult time."
The US military issued a statement saying a coalition helicopter crashed on Tuesday near Salman Pak, on the outskirts of the Iraqi capital, killing two people and injuring 12.
The RAF has 33 Pumas, which are used as battlefield helicopters to transport troops and loads of up to two tonnes by day or night.
The latest deaths bring to 173 the number of British military fatalities since hostilities began in Iraq in March 2003.
