Defence secretary Gavin Williamson is due to see the prime minister this week to lobby for a £2.5bn annual cash boost for the forces

Defence chiefs and ministers are planning to enlist the support of the royal family to persuade Theresa May to give more money to the armed forces.

A plan drawn up to change the prime minister’s mind will see serving and retired officers lobby Prince Charles and Prince Harry in a bid to win the “moral support” of the Queen for Britain remaining a front rank military power.

A senior figure in the Ministry of Defence (MoD) said serving and retired generals and admirals will reach out to the senior royals, including the Duke of Sussex who served in Afghanistan.

They believe the royal family could be influential in the campaign to win over May — although they recognise that the monarch is supposed to remain