Prince William will leave the Army to become a full-time working royal in just 18 months, it can be revealed.

The second in line to the throne will finish his military training at the beginning of 2009 and plans to devote himself to charity work as well learning about the "art of kingship".

He insists, however, that he will not become what he describes as a "full-time meet and greeter", like other members of the Royal Family.

The prince says he wants to take time to learn more about his own strengths and weaknesses as well as the country of which, one day, he will be king.

The revelation comes only days after Prince Harry, "humiliated and depressed" after being banned from going to Iraq, told friends he had decided to quit the Army and become a fulltime working member of the Royal Family.

In advance of William's 25th birthday next Thursday, friends and courtiers have spoken to the Mail with remarkable candour about the future king, painting a picture of a young man who has, at last, come to terms with his destiny.

He is, they say, facing it with optimism, hope and enthusiasm but is determined to do it in his own way and at his own pace.

"He has really matured in the last few years and has come to terms, however reluctantly, with the idea that he is the 'star of the show'," said a senior royal source.

"As a result, he is starting to approach his duties with a grace and humour that is winning people over.

"The idea that he wants to shirk his duties is a gross misrepresentation. The difference is that he doesn't want to feel his entire life is being mapped out in front of him like a set of railway lines.

"Instead, he wants actively to formulate his future plans and play to his strengths.

"He will move fairly quickly into the role of being a working royal but it will be on his own terms."

When he graduated from Sandhurst last December, it was announced that William would follow in his brother Harry's footsteps and become a troop leader with the Blues and Royals.

Unlike Harry, however, William has always known that he will never be a career soldier and it was revealed that he would also spend time learning about the work of the RAF and Royal Navy.

This period of familiarisation will be shorter than expected, however.

After he finishes his tank training at Bovington, Dorset, this summer, William will take part in a handful of exercises with his men and leave the Army by the beginning of next year.

Afterwards, he will spend a few months with each of the other two branches of the military, before moving on.

"William will definitely leave at the beginning of 2009," says an aide. That said, he is totally and utterly committed to what he is doing.

William isn't the kind of man who just wants to put on his medals and parade about. As a future head of the armed forces, he wants to know what these men and women have been through, first hand.

"The key thing for him is to earn his stripes and his credibility."

At the same time, however, Williams plans to start taking on more royal duties, particularly developing his role with the charities of which he is already patron, such as Centrepoint.

He also hopes to take a bigger role at showpiece royal events, such as Remembrance Sunday at the Cenotaph.

By the beginning of 2009, aides believe, he will be taking his first steps toward becoming a full-time working royal.

His integration will, however, be at a "steady" pace, allowing William to develop an understanding of the way the monarchy works, particularly its interaction with the people of Britain.

The source went on: "William is a very thoughtful young man and the key thing for him is to expand the areas that interest him most, with an emphasis on young people, the marginalised and disadvantaged as well as conservation.

"He has inherited his father's passion for the environment and wants to learn more about land and its management.

"That's important for when he eventually takes over the Duchy of Cornwall portfolio, which provides the income for the Princes of Wales."

He has already been flexing his newly-found muscles, sources say, by organising next month's concert in memory of his mother.

"He has led by example, taking a hands-on role organising everything from the guest list to the stage design and programmes," said an aide.

"William has shown he has an innate ability to lead as well as listen and get the best out of people."

Central, friends and aides say, to William's new-found confidence is, surprisingly, his brother Harry.

During their mid-teens the brothers grew apart as their lives took different directions, but in recent years they are said to have become "as thick as thieves".

"They are a great double act partly because Harry is more instinctive, emotional and upfront while William is more cautious. They work well together," said a source close to both brothers.

What of the future king's love-life. He split from long-term girlfriend Kate Middleton in April.

The Mail understands from impeccably placed sources that there have been recent "dalliances" but nothing that could remotely be classed as serious.

"William is wary of becoming close to anyone because of the way in which Kate was so heavily touted as a future bride.

"Plus he is a virile young man who just wants to let his hair down and sample what's out there before he settles down," said a friend. Contrary to reports that William is due to inherit £300,000 from his mother's estate on his 25th birthday, the Prince is not due anything.

He already receives his Army salary, a small allowance from his father, Prince Charles, and what was described last night by a senior royal source as a "modest" annual income from his mother's estate.

The source added: "Neither he nor Harry plan to touch the capital of their mother's estate for several years."