A decision will be made at 8pm UK time on whether the England v Poland World Cup qualifier in Warsaw will go ahead.
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The study by the National Childbirth Trust and National Federation of Women's Institutes found a lack of staff left some women frightened.
Extracts released by 10 Downing Street suggest Mrs May will say the UK will not settle for a "half-in, half-out" policy.
Milder for most of us