Prince William comforted Kate with a touching gesture as the couple paid their respects to the Queen.
Earlier this afternoon, the late monarch left Buckingham Palace for the final time and will now lie in state in Westminster Hall until her funeral on Monday.
The Prince and Princess of Wales paid their tributes to Her Majesty in a sombre ceremony, and William placed a reassuring hand on his wife's back with Kate also seen curtsying towards the Queen.
Both Prince William and Prince Harry were part of the procession that saw the Queen's family march behind her coffin as it was carried to Westminster Hall, with thousands of Brits lining the route in the heart of the capital.
Kate didn't join the procession but left for Westminster Palace by car sitting beside Camilla, Queen Consort.
She was followed by another vehicle that carried the Duchess of Sussex and the Countess of Wessex.
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King Charles III walked in line with his siblings - Princess Anne, Prince Andrew and Prince Edward - and behind them were the Queen's grandsons, Peter Phillips, Prince Harry and Prince William.
They were followed by the late monarch's son-in-law Vice Admiral Sir Tim Laurence, the Duke of Gloucester, the Queen's cousin, and her nephew the Earl of Snowdon.
William and Harry stood together with their wives, Kate and Meghan Markle, during the service having already been seen together at the weekend on a walkabout at Windsor Castle.
The Sussexes stood at the back of the group of royals, with Harry and Meghan behind Kate and William as the couples were seen together for the first time since their surprise appearance at Windsor Castle on Saturday.
The Archbishop of Canterbury then read the opening prayer, which the King led the royals in reciting.
The family stood silently for the short service that the late monarch had put together with the Church of England before she died aged 96.
Cries of "God save the King" were then heard as the King and Queen Consort left Westminster Hall as Big Ben rang out at 3.30pm.
Prince William and Kate left the building side by side, with the Princess of Wales seen rubbing her husband's arm reassuringly.
Mourners have now been let in to see the Queen lying-in-state and the public have been warned the queues could last around 30 hours and are already around three miles long.
The queue, which starts near Lambeth Bridge, began to form earlier this morning.
The Archbishop of Canterbury says it is going to be “very, very emotional” to see the Queen’s coffin as she lies in state in Westminster Hall.
Speaking to PA news agency on Westminster Bridge just before heading to the building for today's service, he said: “I think it’s going to be very, very emotional, as it always is for everyone, wherever they are.
“I was a parish priest for years, the moment when you see a coffin, particularly when you’re one of the family, it pierces to the heart, there’ll be so many people who know exactly what the family are feeling as human beings.
“So, let’s pray for them and keep on praying for them because they need our support.
"They’re having to do all this while they’re grieving when most people are able to take a few days and just think about things.”
