The duke and duchess of Cambridge are to join some 600 former and serving members of the British military on the red carpet Sunday for the London premiere of the Steven Spielberg movie "War Horse."

The movie, which has already moved critics and audiences in the United States since its release two weeks ago, tells the tale of the friendship between a man and horse against the backdrop of World War I.

And with the costs of war at the forefront of people's minds, Kate and William will also announce Sunday that they and Prince Harry are to dedicate much of their fund-raising efforts to helping reintegrate military veterans into civilian life.

The Foundation of Prince William and Prince Harry is to work in partnership with the Forces in Mind Trust, a charity that aims to help veterans who are struggling with issues such as poor mental health, financial pressures, addiction and family breakdown.

Prince William is a qualified Royal Air Force search and rescue pilot, known to his fellow service members as Flight Lt. Wales, while Prince Harry serves in the British Army Air Corps, known to his men as Capt. Wales. Harry recently completed a training exercise flying Apache helicopters in the United States.

The princes' foundation expects to give significant grants to support the work of the Forces in Mind Trust, possibly totaling in the seven figures, a palace spokesman said in a statement, as well as raising the profile of veterans' issues.

The decision was made at the instigation of William and Harry, "who feel that the next major issue facing veterans is their successful reintegration into civilian service," the statement said.

Nick Booth, chief executive of the Foundation of Prince William and Prince Harry, said the foundation is "incredibly excited" about the prospect of helping veterans through the partnership.

Their efforts will be backed up by 35 million pounds ($54 million) in funding from the British National Lottery fund.

The red-carpet premiere of "War Horse," based on a novel by Michael Morpurgo which has also won acclaim as a long-running stage show, comes a day before the duchess turns 30.

A senior palace aide told CNN that the duchess, who is six months older than her husband, will be celebrating her birthday privately and in a low-key fashion.

Catherine took steps to carve out a greater public role for herself this week, announcing the causes she would support as patron.

They included the National Portrait Gallery and an art therapy charity, as well as East Anglia's Children's Hospices and an organization that helps those struggling with addiction.

She will also volunteer with young members of the Scout Association, particularly groups that meet near the home she shares with William in north Wales.

CNN's Max Foster and Laura Smith-Spark contributed to this report.