The Duke of Edinburgh has said he is "disappointed" after being taken to hospital.
Prince Philip had been at Windsor Castle when he was admitted as a "precautionary measure" for treatment of an infection arising from a pre-existing condition, Buckingham Palace have said.
He had been due to accompany the Queen to today's State Opening of Parliament but the Prince of Wales will now do so.
Prince Philip has met dozens of enthusiastic sea cadets in Windsor today.
It's part of his visit to the training ship Windsor Castle in Stovell Road - which is holding an open day.
The Duke of Edinburgh is due to meet sea cadets in Windsor today.
He will visit the training ship Windsor Castle in Stovell Road, which is hosting an open day.
It is the oldest unit in the Sea Cadets Corps and was established back in 1899.
Click for video report. The Queen and Duke of Edinburgh met injured soldiers at a rehabilitation centre today in the region. They visited Headley Court - which treats wounded servicemen and women who've been on the front line. Kate Bunkall reports.
The Queen toured the military's leading rehabilitation centre for injured servicemen and women to learn about their long road back to health.
She was joined by the Duke of Edinburgh and the pair watched as amputee soldiers walked along obstacle courses on artificial legs.
The Queen was making her first visit to Headley Court in Surrey where Britain's battle casualties are sent for intensive rehab once their lives are out of danger.
The Queen and Duke of Edinburgh will visit the armed forces' dedicated rehabilitation centre in Surrey for seriously injured servicemen and women.
The royal couple will tour Headley Court where wounded service personnel are given treatment and support to rebuild their military careers.
The Defence Medical Rehabilitation Centre near Leatherhead is where most combat casualties receive the final stage of their rehab.
It is a sprawling complex of buildings set in the beautiful grounds of a historic building dating back to the Elizabethan period.
Facilities include a hydrotherapy pool, swimming pool, four fully equipped gyms and a state-of-the-art limb-fitting and amputee centre.
The centre has received many royal visits in recent years, most notably from the Duke of Cambridge, who opened some new facilities at the centre in 2010.
During their tour of Headley Court, the Queen and Duke will visit the Waterloo gym, where service personnel who have lost limbs will be put through their paces.
The royal visitors, who will stay for a private lunch, will also meet nursing staff and other patients before the Queen opens a refurbished unit.
The Queen and Duke of Edinburgh will visit the Defence Medical Rehabiltation Centre at Headley Court today.
They will meet with recovering soldiers and open a newly refurbished treatment area.
The Queen and Duke of Edinburgh will attend Margaret Thatcher's funeral, Buckingham Palace said.