With Britain on a 'critical' national security alert Monday, the Transport Police have tightened security around rail and tube stations as City commuters go about their business as usual.

The terrorist attacks on Friday in London's West End and Saturday at Glasgow Airport have once again brought home to City professionals the fact that the threat from al-Qaeda is alive and well. As we come up to the second anniversary of 7/7, police believe that there are around 30 terrorist plots in the works in the UK. 200 suspected terrorist cells are thought to be at work, with no less that 2,000 known suspects currently under surveillance.

Many City workers, of course, face a double-whammy - they are forced to travel to work on public transport (buses and trains are obvious terrorist targets), and actually work in areas considered to be vulnerable to attack. And although the Square Mile is thought likely to be on any terrorist hit-list, the value of an attack there for al-Qaeda would be mostly symbolic. Most agree that the most disruption and human carnage would result from an attack over at Canary Wharf, where many of the City's top investment banks, securities houses, custodians and law firms are now located.

Security in Canary Wharf has, of course, been stepped up. Bus and car searches are likely to cause long tailbacks. More police and security staff are expected to be on the ground. The Wharf itself is no stranger to terror attacks - in November 1992 the Provisional IRA attempted to place a large bomb next to the Canary Wharf Tower, although it was spotted in time by security staff. And 11 years ago the IRA did manage to detonate a large bomb at South Quay, which killed two people and devastated a number of surrounding buildings. The big difference with the IRA attacks, however, is that warnings were usually given and the public were normally quickly able to get away to safety. al-Qaeda, of course, has a different agenda. Over in the US, the Bush Administration has said that it is satisfied with its current terrorism alert level.

So, as we travel to and from work in the days and weeks ahead, let's please heed the wise words of the inimitable Sergeant Phil Esterhaus from the classic US TV cop series 'Hill Street Blues': 'Hey, let's be careful out there'.

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