Packages have been found at army careers offices in Oxford, Slough and Brighton, the South East Counter Terrorism Unit said.
Another suspect package was found in a vehicle which was stopped and searched at 8.30am near RAF Mildenhall in Suffolk, which hosts the US military.
The discoveries come after the army and post offices were put on alert after two "basic but viable explosive devices" were found earlier this week by military recruiters.
St Giles', which runs adjacent to Oxford colleges including Keble and Trinity, has been closed while bomb disposal experts carry out their work.
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