Major American banks in London are tightening up security measures after the discovery of detailed plans for al-Qaeda attacks in the US and UK.
The Home Office insisted there was no specific threat to UK targets
US financial institutions have approached Scotland Yard for advice on how to respond to the heightened alert.
Meanwhile, Conservatives have urged the government to give details of the extent of the threat to Britain.
The Home Office says it is maintaining a state of heightened readiness across the UK.
Plans for terror attacks have reportedly been found on a computer belonging to an al-Qaeda suspect arrested in Pakistan last month.
Shadow home secretary David Davis said he was very worried US plans seemed to be much more advanced than Britain's.
And he called on Prime Minister Tony Blair to spell out the exact threat.
Some US newspapers have questioned the alert, suggesting some of the intelligence behind it is up to three years old.
The New York Times said intelligence and law enforcement officials had cast doubt on the suggestion that a terrorist plot or preparatory surveillance was under way.
But Mr Davis insisted it was essential that more information be shared with the public in the UK and that they be reassured plans to combat the threat were continuing at speed.
He said: "It is astonishing we are getting more information about the risk to Britain from the Americans than from our own government.
"Mr Blair needs to spell out the exact threat to the UK so we are in a
clear position as to where we stand."
Mr Davis said the US was "at a much more advanced stage than us in contingency planning and police presence".
"They also share much more information with their public than our government."
Mr Davis added: "The raw truth is local authorities are under-resourced and Britain does not have one person solely responsible for the job of keeping us safe against
"The sooner we have a Minister for Homeland Security, the better."
The Home Office said in a statement: "We are maintaining a state of heightened readiness in the UK. We are taking feasible precautionary measures to protect British citizens both here and abroad.
"We keep the threat level under constant review. There is a continuing threat that has remained for some time. The public should remain alert but not alarmed."
The Metropolitan Police said they had made no major deployment changes nor stepped up counter-terrorism activity.
"There are only so many things we can do at the level we are at and we are doing all of them," a spokesman said.
But commanders are asking officers to be more visible in order to provide reassurance, Scotland Yard said.
The US said encoded e-mails showed a "credible threat" against targets in New York, Washington DC and New Jersey.
The security alert for financial institutions in those areas of the US has been raised to orange, the second highest level.
Homeland Security Secretary Tom Ridge said buildings specified in the intelligence included the International Monetary Fund and World Bank in Washington, New York Stock Exchange and Citigroup CN in New York and New Jersey's Prudential building.
The proposals were said to be found in e-mails on the computer of Ahmed Khalfan Ghailani, captured after a gun battle in Pakistan on 25 July.
BBC defence correspondent Paul Adams said the detailed planning for attacks on American buildings contrasted with "extremely vague" information on British targets.
But Pakistani officials insist the encrypted e-mails did include threats to unspecified British targets.
Information minister Sheikh Rashid Ahmed said: "We got a few emails from Ghailani's computer about attacks in the US and UK."
Our correspondent said: "It's been reported that American companies with offices in London, like Citigroup and Merrill Lynch, have been advised to tighten security.
"It has also emerged that a Pakistani computer expert arrested in mid-July and now apparently talking to the Pakistani authorities has been a regular visitor to this country."