Businesses in the City of London are being urged by police to report any suspicious activity, amid concern that further terrorist attacks in the UK are being planned in the Square Mile.

The new warning comes after it emerged that one of the UK’s top five banks has been forced in recent weeks to order a review of its security arrangements because of fears that it was being targeted by a terrorist cell.

The FT has learnt that at the end of August two people escaped arrest outside the London headquarters of Lloyds TSB in Gresham Street after being identified on what security sources believe may have been a terrorist planning exercise. The individuals were challenged after being seen taking photographs of the building, but ran towards a car and drove away.

A spokesman for Lloyds TSB said on Friday: “We take the security of our staff and buildings very seriously. We aim to have the best possible security arrangements in place and to maintain a high level of vigilance at all times.”

While the police subsequently told the bank there was no intelligence to suggest that its building was a target, the bank confirmed last night that following the incident it had raised the level of security in all its buildings in London.

The City of London Police on Friday said they could not comment directly on an alleged incident as a matter of policy. However Frank Armstrong, the assistant commissioner, confirmed that police were on the look out for “hostile reconnaissance” and urged businesses to report any similar sightings.

“Hostile reconnaissance” is a well known terrorist tactic whereby a range of potential targets are observed and information gathered prior to a more specific operation being planned or attack being ordered.

The UK threat level has remained officially unchanged since being lowered from critical to severe on July 4, meaning that future terrorist attacks are highly likely but no longer thought to be imminent.

Security in the City of London has been stepped up since the last terrorist attack-by the IRA-in Bishopsgate in 1993.

MI5, the security service, warned earlier this year that terrorist “attack planning” would increase in 2007.

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