At least 18 contractors working in the area near a chemical leak have complained of respiratory distress, police said, adding that residents in the area had been advised to remain indoors, with their windows shut.
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The PM promises to double to 200,000 the number of homes built under an existing scheme aimed at easing Britain's housing shortage.
The Met Office has weather warnings in place for snow and ice until 10am in the north and west.
The plan, which would effectively strip banks of the benefit of recent corporation tax cuts, forms part of the party's election manifesto.