A female police officer has been shot dead and another seriously injured as they responded to an armed robbery in the centre of Bradford.
The gunmen are still at large, with surrounding roads closed and dozens of armed police patrolling the area in an effort to track them down.
The officers were called to Morley Street at 1525 GMT to deal with a robbery at a travel agent.
Tony Blair said he was "shocked and saddened" by the death.
One eyewitness told BBC News the robbery was at Universal Express travel agent.
Members of staff at the family-run business were assaulted and cash taken.
It is thought the officer who died was in her mid-30s and may have been a "probationer" - of less than two years' service.
A Downing Street spokesman said Tony Blair was "shocked and saddened" to hear of the shootings in Bradford and "his thoughts go out to the family and friends of the officers involved and to West Yorkshire police force".
Home Secretary Charles Clarke said: "While incidents of this sort are mercifully rare, they serve to remind us of the risks and dangers police officers face every day in their work to uphold the law and protect the public."
The incident comes less than three years after 36-year-old Tasawar Hussain was shot dead as he tried to help police stop an armed robbery at another travel agent in Bradford.
Two men were later jailed for life for his murder and Mr Hussain was given a posthumous bravery award.
Two years ago Pc Ian Broadhurst from West Yorkshire police was shot dead in Leeds.
Former US Marine David Bieber was given a life sentence for the execution-style killing at Newcastle Crown Court in December 2004.
The last female officer killed in the UK was 29-year-old Pc Alison Armitage in March 2001 - she was run over and killed by a car thief in Oldham Lancashire.
Before Friday's incident, the last police fatality was Det Con Michael Swindells who was fatally stabbed by a paranoid schizophrenic in May 2004 in Birmigham.