A group calling itself the IRA has claimed responsibility for suspect packages sent to armed forces recruitment centres last week. 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.
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The 24-year-old was unarmed and the incident is not being treated as terror-related.
Some platforms have been taken out of service with disruption expected to continue until the end of the day.
The Met Office has extended its weather warnings for snow and ice to cover vast swathes of the country, including London and the South East.