In the Second World War he served as a tank commander in the Grenadier Guards and was awarded the Military Cross.
He chaired the Lancaster House talks in 1979 that brought majority rule to Zimbabwe but resigned after the invasion of the Falklands in 1982 – saying there had to be a "political sacrifice".
The Dean of Westminster Abbey, John Hall, said:
"We are inspired by his bravery and modesty, and by his example of high honour and public duty.
"So, let us pray today that our land will never lack men and women willing and able to commit their lives to the public good and to the service of Crown and country, and that we ourselves may continue to guard what is right and fight for what is just."