The brothers will be joined by their cousin Peter Phillips, Vice Admiral Sir Timothy Laurence, the Duke of Gloucester and the Earl of Snowdon.
The Queen Consort, Princess of Wales, Duchess of Sussex and Countess of Wessex will travel to Westminster by car.
It will be the first time the late Queen’s grandchildren and their spouses have joined in the ceremonial mourning.
At Westminster Hall, Elizabeth II will lie in state until the Monday morning of her funeral, with an almost constant stream of members of the public passing her coffin.
Some 400,000 people are expected to attend, amid growing concern that five-mile queues requiring a 30-hour wait will prevent children and the elderly from playing their small part in history.
The royal princess accompanies her mother on the last flight
On Tuesday, the Princess Royal accompanied her mother on her final flight, departing at 5.42pm in an RAF Globemaster C-17 military transport aircraft from Edinburgh to West London, while the Queen departed from Scotland for the last time.
The princess said it had been an “honour and a privilege to accompany her on her final journeys”, having been with the Queen in the last 24 hours of her life.
“Seeing the love and respect shown by so many on these trips has been both humbling and uplifting,” he said.
Hundreds of thousands of people across Scotland turned out to pay their personal respects, with 33,000 queuing for up to 12 hours to pass by as the Queen lay in repose at St Giles’ Cathedral and toured the streets.
At 4.20pm on Tuesday, the coffin was carried from the cathedral to the sound of a lone piper to the waiting hearse, followed by soldiers from the Royal Regiment of Scotland.
Citizens lined the roads to the airport, some doffing their hats as the hearse passed them, others watched in silence or filmed with their mobile phones scenes that encapsulated the mourning of the age modern
At Edinburgh Airport, the Royal Regiment of Scotland formed a guard of honor of three officers and 101 soldiers, with an aircraft carrier group provided by the Queen’s Color Squadron, Royal Air Force.
The plane, recently used for Operation Pitting to evacuate thousands of people fleeing the Taliban, took off at 5:42 p.m.