Britain and the United States are planning an exercise to combat a simultaneous terror attack in both countries, a scenario ministers says their security services believe is a real possibility.
British Home Secretary David Blunkett flies to Washington on Sunday for talks with senior figures in President George W. Bush's administration on setting up a simulation.
Blunkett told Sky Television there was already unprecedented co-operation between London and Washington but both governments had to work out where and how militants might strike next.
"We have to have every eventuality in mind and one of those is that they would want to launch a simultaneous attack on the United States and the United Kingdom," he said.
"Our security and intelligence services put to us that this is something we should be prepared for. "To do so, we do need a fully fledged exercise on the ground...where we could test out what would happen in reality."
A number of flights across the Atlantic have been cancelled this year on security advice.
Last year Britain held a simulation of a chemical attack on an underground train in the heart of London.
Officials said Blunkett would meet Bush's security advisers to discuss a joint exercise and sharing information in areas like aviation security.
"The aim is to ensure everything is in place at the highest level to enable that essential work to continue," a Home Office spokeswoman said.
Blunkett had also been due to meet Attorney General John Ashcroft for talks on tackling organised crime and how London and Washington can work together to shut down illegal internet pornography sites.
But Ashcroft was admitted to hospital on Thursday with severe gallstone pancreatitis and remained in intensive care at the weekend.
Blunkett, who revels in his reputation as a hard-liner, will make a speech in the United States on Monday on balancing national security interests and democratic freedoms.
He has recently floated ideas like lowering the burden of proof in court cases brought against terror suspects, allowing bugged telephone conversations to be used as evidence and using special judges to hear trials involving sensitive intelligence.
Blunkett has also pledged to retain emergency powers to imprison foreign terror suspects indefinitely without charge.