The plane crashed north west of Baghdad en route to Balad
At least nine British soldiers died when a transport plane crashed in Iraq.
Prime Minister Tony Blair paid tribute to the troops killed, saying: "This country and the wider world will never forget them."
The C-130 Hercules based at RAF Lyneham in Wiltshire, crashed about 40 kilometres (25 miles) north west of Baghdad, the Ministry of Defence said.
The final death toll from the crash, which happened at 1725 local time (1425 GMT), may rise to 15, say sources.
The plane was en route from Baghdad to Balad, north-west of the capital, which houses one of the largest US airbases in Iraq.
There are no British troops usually based in Balad and the purpose of the flight was unclear.
US and UK forces have secured the crash site and are trying to establish the cause of the crash.
Wing Commander Nigel Arnold of RAF Lyneham said they were in the process of contacting the families of those involved.
"Until that's done I am afraid we will not be releasing any details of the crew," he said.
"The welfare of these families is our priority at the moment."
He said it would also be wrong to speculate on the cause of the crash at this stage, but they would carrying out a thorough investigation.
US President George W Bush has also paid tribute to the soldiers killed, as polls opened across Iraq for its first free election in more than 50 years.
"Some Iraqis were killed while exercising their rights as citizens," he said.
Wing Commander Arnold said an investigation would take place
"We also mourn the American and British military personnel who lost their lives today. Their sacrifices were made in a vital cause of freedom, peace in troubled region, and a more secure future for us all."
Hercules planes have an excellent safety record, according to Paul Beaver of Jane's Defence Weekly.
The BBC's Clarence Mitchell said the weather in the region was fine and, given the good safety record of the plane, the "chances of it being hostile action against it are increasing".
The US-built troop carrier is used by both British and US forces.
British military chiefs have used the plane as a "workhorse" to transport troops, fuel, ammunition and humanitarian relief.
It is the favoured aircraft of special forces (SAS) and is regarded as being especially of use in Iraq for its ability to land in desert conditions.
An RAF Hercules crashed in Albania in 1999 because the load was not properly strapped down, he added.
The MoD has a phone number for concerned relatives to call: 08457 800 900