Counter-terrorism police are investigating reports of suspect packages being found at army careers offices.
Packages have been found in Canterbury, Oxford, Slough and Brighton, while similar packages were found earlier in the week in Aldershot, Reading and Chatham, the south east counter terrorism unit said.
Police officers consider the situation a "low-level threat" but "viable", sources said.
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It will be increasingly humid and thundery with highs of 30C or more today.
Drink lots of water, stay out of the sun during the middle of the day, and take particular care of vulnerable people.