Britain will build what it is calling a new "national flagship" vessel to host trade events and promote its post-Brexit interests around the world, Prime Minister Boris Johnson announced on Saturday.

Fishing boats moored at La Collette Marina in St Helier, Jersey, on Saturday, May 8. Photo: VCG

The ship will provide a global platform for high-level trade negotiations as well as British businesses' products, his office said, as the UK seeks new trading ties after leaving the European Union in 2020.

It will also be expected to play a role in delivering the country's foreign and security policies, including by hosting summits and other diplomatic talks.

It will be the first so-called national flagship in service since 1997, when the Royal Yacht Britannia was decommissioned.

However, Johnson insisted the new vessel's role will be "distinct" from those of forerunners, "reflecting the UK's burgeoning status as a great, independent maritime trading nation."

"Every aspect of the ship, from its build to the businesses it showcases on board, will represent and promote the best of British - a clear and powerful symbol of our commitment to be an active player on the world stage," he said in a statement. 

The ship's construction is expected to begin in 2022 and be completed within four years, with its costs confirmed following a competitive tendering process, according to Johnson's office.

The vessel, yet to be named, will be crewed by the Royal Navy and earmarked for around 30 years' service.

The government is likely to face calls to name it after Prince Philip, Queen Elizabeth II's late husband and a former navy commander, who died in April aged 99. Britain formally left the EU after nearly five decades of membership in January 2020, and quit its single market and customs union at the start of 2021.