A group calling itself the IRA has claimed it was behind devices sent to armed forces recruitment centres last week, Scotland Yard said.
Scotland Yard said a group using a recognised codeword had contacted a media outlet in Northern Ireland on Saturday.
Four suspected explosive devices were discovered at Army careers offices in Oxford, Brighton, Canterbury and the Queensmere shopping centre in Slough last Thursday.
This followed packets sent to Aldershot, Hampshire, on Wednesday and another two on Tuesday to an armed forces careers office in Reading, Berkshire, and the Army and RAF careers office in Chatham, Kent.
One of the packages bore a Republic of Ireland postmark and Downing Street said the small but potentially viable devices bore "the hallmarks of Northern Ireland-related terrorism".
Scotland Yard said: "We are aware of the claim of responsibility for the devices that were sent to Army recruitment centres in England last week.
"The claim was received on Saturday February 15 by a Northern Irish media outlet using a recognised codeword. The claim was allegedly made on behalf of the 'IRA'.
"The public is urged to remain vigilant and report anything suspicious to the Anti-Terrorist Hotline, 0800 789 321."
The IRA disbanded in the years after it declared an end to its armed campaign in 2005, but a group calling itself the New IRA formed just before the London Olympics in 2012 and was linked to letter bombs sent last autumn.