Buckingham Palace said on Wednesday morning that Queen Elizabeth pulled out of a planned engagement in Northern Ireland and had been advised to rest on medical advice.
But royal officials conceded late on Thursday that she had in fact had "some preliminary investigations" in hospital on Wednesday, and stayed overnight, after The Sun newspaper broke the news.
BBC royal correspondent Nicholas Witchell said royal officials "have not been giving us a complete, reasonable picture of what has been occurring".
"Rumours and misinformation proliferate (and) thrive when there is an absence of good, proportionate, trustworthy information.
"We must hope that we can rely on what the palace is now telling us," he added, calling assurances that the queen was in good spirits "a handy phrase that the palace dusts off at moments such as this".
Richard Palmer, royal correspondent at the Daily Express, also called the phrase a "palace cliche".
"Royal sources had been keen to encourage the impression that she had just overdone it but may struggle to convince the public now," he said.
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