Pictured: Blair supports frail Lady Thatcher at service to honour Northern Ireland veterans

The Prince of Wales, who later hosted a reception for the veterans, the Duchess of Cornwall and Conservative leader David Cameron were also among the guests at the service at St Paul's Cathedral.

Servicemen stood guard outside as dignitaries filtered into the cathedral.

Lady Thatcher, whose entire prime ministerial career was played out against the backdrop of 'the Troubles', was dressed in sombre tones.

Helping hand: Former prime minister Tony Blair assists Margaret Thatcher at the service to commemorate the end of Operation Banner

The Duchess, wearing turquoise, smiled to photographers.

Dignitaries including Chief of the Defence Staff Air Chief Marshal Sir Jock Stirrup and former Prime Minister Tony Blair also attended the service.

The deployment, known as Operation Banner, was the Army's longest continuous campaign and saw more than 300,000 military personnel serve in Northern Ireland.

Troops were first sent in during 1969 as violent unrest escalated on the streets, and over the following decades 763 members of the armed forces were killed by paramilitaries.

Operation Banner formally ended last year at midnight on July 31 and at the time, Armed Forces minister Bob Ainsworth said: 'The impact of the commitment since 1969 has been considerable on both the military themselves and on the MoD civilians supporting them.'

Iron lady: Mrs Thatcher's term in office was blighted by 'The Troubles' in Northern Ireland

After the service, about 700 veterans marched from the cathedral to Guildhall for the reception and the prince took the royal salute.

The commemoration service began with a fanfare of trumpeters.

In a short statement Prince Charles described how events in Northern Ireland had touched his own family.

He said: 'The effects of the situation in the province have been felt by many people, both in the United Kingdom and beyond, and I can only too well imagine how shattering it has been for those who have lost loved ones during these difficult times, having myself experienced the loss of my great uncle, Lord Mountbatten, to a terrorist bomb in 1979.'

Mountbatten was killed by a provisional IRA bomb detonated while he and his party were on a sailing boat off the coast of the Republic of Ireland.

Royal approval: Prince Charles and Camilla, Duchess of Cornwall, leave St Paul's Cathedral after the service

Tough times: Prime Minister Gordon Brown chats with the soldiers as he arrives at St Paul's Cathedral,

The prince added: 'It is vital that we, as a nation, remember all those who have made the ultimate sacrifice in bringing greater peace and security in Northern Ireland.'

The moving church service saw hymns sung and included prayers for those who served in Northern Ireland.

One praised the armed forces for their courage, fortitude and dedication to duty.

While another asked that the 'building blocks of peace may be strengthened and secured'.

The Last Post was sounded and echoed around the cavernous interior of St Paul's and silence was kept for a few moments after it ended to remember all those who died.

A few moments later the Operation Banner memorial candle was lit by Mary Moreland, who lost her husband to a terrorist attack in Northern Ireland in 1988.

Private John Moreland was a part-time soldier in the Ulster Defence Regiment and was killed while doing his civilian job as a coal merchant in Downpatrick, County Down.

The candle was lit as part of the Act of Remembrance and symbolises all members of the armed forces killed over the last 38 years.

Salute: The Lord Mayor of London and The Prince of Wales salute the Servicemen and Women at St Paul's Cathedral