On Wednesday, Buckingham Palace said the Queen was in 'good spirits'. But it is believed doctors ordered her to rest after a slew of work commitments since returning from Balmoral at the start of October.
Last week she was forced to start using a walking stick in public for the first time, a stark reminder of her advancing years.
And on Tuesday night she hosted a major summit for global investors and politicians at Windsor Castle, standing on her feet and shaking hands for around 40 minutes.
Royal sources said there was no cause for concern, and stressed the decision to cancel her trip was purely a 'precautionary' measure.
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