Three people were slightly injured when the small device exploded without warning at the height of the Saturday afternoon shopping rush at Ealing Broadway, west London.
The homemade bomb was in a flowerbed in an alley to the side of a Barclays branch, just yards from three cash dispensing machines. Shoppers were evacuated from the scene within minutes and anti-terrorist branch officers spent almost four hours combing the area for forensic evidence.
It was the second time in three days that a bomb had exploded in west London, but police ruled out suggestions that it could be the work of the IRA, owing to the small size of the improvised device. Yesterday, officers were examining the possibility of a link to the terror campaign against Barclays.
Earlier this month it emerged that a blackmailer had been terrorising Barclays for 16 months. Calling himself Mardi Gra, the extortionist has posted or planted more than 25 devices. Until Saturday no one had been badly hurt because the devices were all missing a part of the detonator.
The blackmailer, described by detectives as an obsessive man, appears to have taken a great pride in his work - bombs have been meticulously packaged and he has used distinctive logos and wording. He first struck in December 1994, when he sent six devices to Barclays branches in north- west London. Only one went off, burning the hands of the woman who opened it.
The devices have been fitted into video boxes and books, and featured shotgun cartridges and bullets designed to explode when opened. Most devices were sent to addresses in London, direct to the bank, its officials, companies connected to them or placed in telephone boxes outside banks.
Last night a Barclays spokesman said that the bank had received no further information from the police as to the identity of the bomber. "It is of great concern to us. Obviously we are sending out regular advice to staff and this will continue."