A terror attack on London's financial district is only a matter of time but just half of firms have disaster contingency plans in place, a senior police officer has warned.

James Hart, City of London Police Commissoner, said several locations in the heart of the Capital's financial centre have already been staked out by potential terrorists since the New York attacks on September 11 2001.

Speaking in the Financial Times, Hart claimed radicals had focused on sites where bombings would result in mass murder and full-scale disruption including businesses and prominent buildings.

Hart pointed out the City of London has been a target for terrorist attacks for over 30 years with several explosions orchestrated by the IRA.

"If you want to hurt the government, hurt people at the same time, and you want to cause maximum disruption…where better to hit than at the financial centre?", he said.

"I think [another attack] is a question of when rather than if."

Although last month's London bombings have prompted many firms to renew disaster response planning, Hart estimated only 50% of businesses have such plans in place.

The commissioner blamed the shortcomings on chief executives and business boards.