The Prime Minister's plans to redraw Westminster's constituency boundaries and cut the number of MPs at the next general election have been rejected in the Commons by 334 to 292, majority 42, following a major split in the coalition Government.
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It reportedly threatened attacks on planes "flying from Heathrow to the US" across the July 4 US Independence Day holiday weekend.
Accompanied by the Duke of Edinburgh, the Queen will attend the opening ceremony of the fifth session in the Holyrood chamber.
Ms Leadsom has received the backing of Ian Duncan Smith, the former work and pensions secretary.