During the following decades the charity has helped members of the armed forces from every major conflict.
Retired Lieutenant General James Bashall, the RBL’s national president, said the centenary was a “very proud moment” for all those associated with the legion and it was an honour to have their patron the Queen and Anne as guests at the service.
He said: “In our centenary year, we remain committed to our mission to ensure that those who have given so much for their country get the fair treatment, support and recognition they deserve.
“And, as we look ahead to the next century, we invite the next generation to continue our vital work in the years to come.”
General Sir Nick Carter, Chief of the Defence Staff, will be among the congregation for the service and the outgoing head of the armed forces will be joined by members of the military, veterans and their families from the UK and the Commonwealth.
Posthumous Victoria Cross
RBL supporters, volunteers and partner organisations will also be sat in the pews for the service led by the Dean of Westminster Dr David Hoyle.
Readings will be given by C/Sgt Beharry, Sir Nick and Sara Jones, whose husband Lieutenant Colonel Herbert “H” Jones was awarded a posthumous Victoria Cross for his bravery at the Battle of Goose Green during the Falklands Conflict.
The Princess Royal will give a scripture reading on the theme of service, and there will be a rededication from the RBL’s national president, reaffirming the legion’s commitment to its work.
Naomi Hall, an RAF and Afghanistan veteran supported by the RBL as she recovered from physical and mental health injuries, will also give a reading.
Ms Hall said: “It is a great honour to be asked to help mark the centenary of an organisation that has done so much for me and my military comrades for the last 100 years, and a privilege to do so in the presence of Her Majesty the Queen.
“When I was at my lowest ebb, the RBL was there for me and has been ever since. My road to recovery has been a long journey and I could never have imagined it would lead to this day.”