The Queen will be guest of honour at the commissioning of the UK’s new aircraft carrier – HMS Queen Elizabeth.
The monarch will board her namesake ship in Portsmouth to see the Royal Navy White Ensign raised on the vessel for the first time.
Also present will be Princess Anne, Prime Minister Theresa May and First Sea Lord Admiral Sir Philip Jones.
The £3.1bn carrier and its sister ship HMS Prince of Wales are the most expensive in the Royal Navy’s history.
The navy initially estimated both ships would cost £3.5bn to build but the total figure was revised to £6.2bn.
About 3,700 guests are due to attend the event, which comes more than three years after the vessel’s official naming ceremony in Rosyth when the Queen broke a bottle of whisky on its hull.
During the ceremony, the commissioning warrant will be read, and the Blue Ensign, which has been flying from the ship until it is formally handed over to the Royal Navy, will be replaced with the White Ensign, raised by 20-year-old Able Seaman Ellie Smith from Hull.
‘A potent symbol?’
By Jonathan Beale, Defence Correspondent
This is a big day for the Royal Navy. A chance to look to the future and, at least for a moment, forget about recent defence cuts and fears of even more.
After successfully completing her sea trials HMS Queen Elizabeth will be commissioned into service. For the first time she’ll raise the White Ensign – officially becoming a Royal Navy Warship. But, this is still another milestone not the end of her journey.
Flight trials will begin next year and her first proper deployment with jets on board isn’t planned until 2021. It’s also still not clear how many of the new F35 jets she’ll carry.
Certainly fewer than the 36 she was built for, with each jet costing around £100m. The Royal Navy believes the carrier – the first of two – will be a potent symbol of British military power. But it’s already struggling with limited resources.
The Queen will also review the first row of the royal guard and say a few words.
Capt Jerry Kyd called the ceremony the “culmination of a number of years of real excitement”.
He said: “The first sailing from Rosyth was only nine months ago, we have come a long way.
“The first entry into Portsmouth was in the summer and here we are today accepting the ship into Her Majesty’s fleet formally.
“So, it is right at the top, it is the latest milestone, many more to come, but hugely exciting and a very proud day.”
The 900ft-long (280m) carrier cannot currently deploy planes but F-35B Lightning fighter jets are due to make their first trial flights from the carrier’s deck next year, with 120 air crew currently training in the US.
Preparations for the arrival of the flagship of the fleet and its 700-strong crew led to more than 20,000 items, ranging from a human skull to sea mines, dredged up from Portsmouth Harbour.
The Ministry of Defence said specialist dredging vessels had removed 3.2 million cubic metres of sediment – equivalent to 1,280 Olympic swimming pools – during the dredging operation carried out to deepen the harbour mouth to enable the Queen Elizabeth to reach Portsmouth naval base.