Staff from the Royal Estate at Sandringham are among those who have been called to London this morning for what one source described as a 'highly unusual' meeting.

Workers in the Royal Household are occasionally summoned to Buckingham Palace for meetings, and another insider told a news agency there was 'no cause for alarm'.

But the short notice at which the meeting appears to have been called has sparked a frenzy of media speculation. While the subject of the meeting has not yet been revealed, one source said it was 'something major'.

It is understood staff will be addressed at 10am by the Lord Chamberlain and the Queen's private secretary Sir Christopher Geidt.

Dickie Arbiter, the Queen's former press secretary, said the announcement could concern the £369m refurbishment of Buckingham Palace.

Peter Hunt, the BBC's royal correspondent, tweeted: 'The BBC understands there is no cause for concern regarding the health of either the Queen or Prince Philip.'

Yesterday the Queen and Prince Philip both fulfilled engagements.

The Queen met Prime Minister Theresa May at Buckingham Palace to formally agree the dissolution of parliament ahead of the general election, while Prince Philip attended Lord's Cricket Ground to open a new stand.