The only conspiracy theory surrounding the Royal Family yesterday was whether Kate Middleton’s presence at Sandhurst to see her boyfriend, Prince William, pass out as an officer of the British Army was a signal that the couple plan to marry.

Three generations of the family attended the Sovereign’s Parade at Sandhurst in Surrey.

The Queen, the Duke of Edinburgh, the Prince of Wales and the Duchess of Cornwall were in the VIP section, but all eyes were on the general stand, where Miss Middleton sat.

She was not hard to spot in a bright red coat and black broad-brimmed hat, necessary protection against the bitter wind.

Although royal aides made the point that she was there in a purely private capacity as William’s girlfriend and was not an official member of the royal party, the couple’s closeness fuelled speculation that they will one day tie the knot.

Miss Middleton, who met the 24-year-old Prince when they were studying history of art at the University of St Andrews, in Fife, spent the ceremony grinning and chatting to her parents. As William passed by carrying an SA80 rifle, she pointed him out excitedly.

ITN used a lip reader to translate Miss Middleton’s comments to onlookers — who told them that she said: “I love the uniform. It’s so so sexy.”

Prince William now holds the rank of Cornet — equivalent to 2nd Lieutenant — in the Blues and Royals, the same Household Cavalry regiment as his younger brother, Prince Harry, who passed out of Sandhurst in April.

The Prince wore a red sash, which marked him out as an escort of the sovereign’s banner. He looked sombre, but broke into a grin when the Queen stopped to say a few words as she walked down the lines of officer cadets.

Last night he and Miss Middleton were due to attend the traditional all-night ball that follows the passing-out parade. At the stroke of midnight the newly commissioned officers peel off black tape concealing their badges of rank.

Prince William’s big day was a chance for the Royal Family to move on after the publication of the findings of the official inquiry into the death of his mother, Diana, Princess of Wales.

The report, written by the former commissioner of the Metropolitan Police Lord Stevens of Kirkwhelpington, found no evidence to support any of the conspiracy theories surrounding her death in a car crash in Paris in 1997.

In another sign of how much things have changed in the past ten years, the parade was the occasion of the unveiling of the first group portrait of the Royal Family to include the Duchess of Cornwall.

The picture, by Sergei Pavlenko, records the moment that the Royal Family gathered to attend the passing-out parade of Prince Harry.

The Queen was clearly delighted with the new painting when she saw it for the first time yesterday. It hangs in the grand entrance hall of Old College and was completed just in time for the commissioning of Prince William.

Prince Harry was a notable absentee yesterday, as he was on duty in Windsor. His brother is expected to follow in his footsteps by completing a three-month junior leaders’ course at Bovington, Dorset, the home of the Royal Armoured Corps.

No decision has been taken on whether either of the Princes will be allowed to serve in Iraq or Afghanistan. But that possibility cannot have been far from the Queen’s mind when she told her new officers: “For those who are to be commissioned today, a great deal will be expected of you.

“You must be courageous yet selfless, leaders yet carers, confident yet considerate. And you must be all these things in some of the most challenging environments around the world.”

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