For six months, Richard Taulbut and Robert Jolly have had to keep a very big secret.

They have not even told their families and close friends.

But today it will finally be revealed.

The two men designed and carved the inscription on the memorial to Diana, Princess of Wales in Hyde Park, London, which has remained under wraps even after the stone watercourse was uncovered last week.

The Queen was unveiling the lettering at 11am today.

Running along the inside of the oval fountain, the 4in letters took three weeks to carve into the granite.

A special finish on the inscription means sunlight will reflect off the lettering.

Mr Taulbut, from Worthing, said: "If I had told anyone what was said before it was revealed they would have put me in the Tower of London. Or they wouldn't have paid me. It's a strict part of our contract not to reveal what has been written."

The 36-year-old, who works with fellow stone-carver Mr Jolly, 47, from Littlehampton, at The Stone Studio in Middleton, near Bognor, said it was the hardest job he had done.

Mr Taulbut said: "The memorial is made out of granite, which made it a challenge. The stone is so hard and so coarse that it's very difficult to get smooth lines on the outside of the letters.

"All the lettering we do is hand drawn and carved. We also designed the lettering. The brief we were given by the architects was to keep it simple but elegant, like Diana.

"It was hard physically, because of the granite, but also because it's such an important thing to work on. Keeping it a secret has also been hard.

"Most big jobs would take one to two weeks. This took us three."

The 260ft by 150ft oval-shaped feature, designed by American landscape architect Kathryn Gustafson, is based on a large oval stone ring and is said to illustrate changes in Diana's life.

Water pours into the structure from the top of a hill at the Serpentine Bridge and runs in two directions.

In one area it is choppy and fast-flowing, reflecting Diana's turbulent marriage and public life.

A calm pool at the base is said to symbolise the peace and serenity she found.

The Government-commissioned memorial took almost seven years to complete after being held up by bureaucratic wrangles and went £600,000 over its £3 million budget.

Mr Taulbut, who has been invited to the unveiling, said: "I think it reflects the princess. It's modern rather than traditional, which reflects the modern approach she brought to the Royal Family."

Chichester-based Cathedral Works Organisation was in charge of the overall construction of the project.

The Queen, Prince Charles, Princes William and Harry, Prince Philip and Diana's brother Earl Spencer will all attend the unveiling.

It is not The Stone Studio's first royal commission. Mr Jolly has hand-carved memorials for the Queen's corgis.

The company has also completed inscriptions and gilding on the Royal Albert Hall war memorial in London, the Falklands War memorial in Portsmouth and work at Blenheim Palace and Chichester Cathedral.