Shopping centres and major retail destinations were on high alert this morning after Irish terrorists threatened to bomb London as the Queen visits Dublin today.

Businesses in the capital were emailed about the coded threat early yesterday.

Counter-terror police told them the warning had been delivered by the Real IRA and was being treated as credible. Security sources said that police fear they may try to inflict mass casualties, said The Daily Mirror, which cited retail destinations such as Westfield, Harrods and Selfridges as possible targets.

The bomb warning sparked a huge security alert in central London after the caller claimed a device had been planted and was primed to explode.

Scotland Yard urged Londoners to go about their normal business but to look out for unusual activity or behaviour as police took the unusual step of publicising the threat.

The threat, which was “not specific to time or place”, was issued at around 8pm on Sunday and raised the terror threat status for the capital to “severe”, the second highest level.

Shops, hotels and other businesses were contacted by anti-terror police warning them to be on high alert.

The UK mainland has not experienced an Irish republican attack since car bombs exploded at BBC Television Centre and Ealing Broadway station in London in 2001.