A letter sent to the US senate office has tested positive for the poison ricin, reuters has reported.
US senator Durbin said he was briefed by the FBI on an envelope that test positive for ricin.
The senator said he believed the letter was just one of suspicious letters that have been found.
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Ms Rayner and the Labour leader will visit Blackpool on Tuesday to outline their proposals.
The far south and the northwest of England will be driest.