King Charles III will hail the value of traditional alliances and international friendships at a time of "increasing pressures of conflict" in his Commonwealth Day message.

The annual celebration of the Commonwealth, to be marked on Monday with a service at Westminster Abbey, will see the biggest gathering of senior royals since the arrest of his brother, Andrew Mountbatten-Windsor.

There will also be contributions at the service from performers including former Spice Girl Geri Halliwell-Horner and Strictly Come Dancing's Oti Mabuse.

Against a backdrop of war in the Middle East and Ukraine, the King will commend the unifying force of the Commonwealth, a voluntary association of 56 countries.

"It is often in such testing moments that the enduring spirit of the Commonwealth is most clearly revealed," the King's message will say.

The King, who has campaigned for many years to protect the environment, will also reflect, in his written message, on pressures from "climate change and rapid transformation".

He will say the Commonwealth is a "force for good - grounded in community, committed to the kind of restorative sustainability that has a return on investment, enriched by culture, steadfast in its care for our planet, and united in friendship and in the service of its people".

The reference to the significance of climate change is in contrast to the approach of the administration of President Donald Trump in the US, who last month reversed a ruling that underpinned federal attempts to curb greenhouse gases.

There have been reports that the King could carry out a state visit to the US next month, but nothing has so far been confirmed.

The annual Commonwealth service at Westminster Abbey is usually attended by senior royals, political and religious leaders, and representatives of Commonwealth nations.

The multi-faith service, held since 1972, is a mixture of music, prayers, dance and readings, reflecting the diversity of the Commonwealth. But unlike in recent years, the service is not being broadcast live by the BBC because of "funding challenges".

Also taking part will be dancers from the Royal Ballet School, and singers Indigo Marshall and Tarju Le'Sano, and there will be a new piece of music by composer Rekesh Chauhan.

The Commonwealth's Secretary General, Shirley Ayorkor Botchwey, will speak at the service.

Among the future issues facing Commonwealth leaders will be whether Mountbatten-Windsor should keep his place in the line of succession, after his arrest on suspicion of misconduct in public office last month, and in the wake of revelations from the Epstein files.

Canada's Prime Minister, Mark Carney, added his voice to calls for his removal from the line of succession, following similar calls from Australia and New Zealand.

Andrew remains eighth in line to the throne, and removing him would require legislation in the UK and the approval of the 14 Commonwealth realms that have King Charles as head of state.

Mountbatten-Windsor has denied any wrongdoing from his association with Jeffrey Epstein.