Buckingham Palace insists The Queen Consort has made an "excellent recovery" from her second bout of Covid-19.
But Her Majesty's last-minute absence from a long-planned engagement with The King has inevitably raised eyebrows.
Queen Camilla has not been seen in public for almost two weeks and was forced to pull out of several engagements after suffering "cold-like symptoms" related to coronavirus.
Her Majesty is fully vaccinated against the disease, it is understood.
The Queen Consort is determined to attend the rescheduled literary reception tomorrow
Chris Jackson
It is thought The Queen Consort was incredibly disappointed to cancel her literary reception at Clarence House, which would have celebrated the second anniversary of her highly successful Instagram "Reading Room", which encourages more people to read and boasts 155,000 followers worldwide.
Championing literacy in both children and adults is one of Her Majesty's big passions, and she "understands the importance of literacy in creating life chances".
A Buckingham Palace spokesperson cited the rescheduling of the Clarence House literary reception as a reason for pulling out of today's visit to The Felix Project in East London - which is the capital's largest food redistribution charity and tackles food poverty and waste.
The spokesperson said: "Her Majesty has made an excellent recovery from last week’s illness. However, to allow for additional preparation ahead of Thursday’s rescheduled literary reception and other commitments, Her Majesty did not attend today’s Felix Project engagement. His Majesty’s visit remained unaffected."
It appears The Queen Consort is determined to attend the rescheduled literary reception tomorrow - a royal source implied Her Majesty dislikes letting people down.
The fact King Charles has not changed any of his public plans so far suggests there is absolutely no cause for alarm, but all eyes will be on tomorrow to see if The Queen Consort is well enough to host a reception in her London home.
