With fuel tankers voting overwhelmingly in favour of strike action, the Defence Secretary Phiiip Hammond says "if there is industrial action my colleagues in government would be looking to the military for some support."
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The major party leaders' campaigns for the General Election will remain suspended on Wednesday following the Manchester terror attack.
The threat level has been raised to critical by MI5's Counter-Terrorism Analysis Centre.
Prime Minister Theresa May announced the decision from Downing Street.