Diana was bugged by secret service in US

She was even forced to abandon a planned holiday with her sons in the US with tycoon Teddy Forstmann on advice from secret services, who passed on their concerns to their British counterparts.

Both US and British intelligence then forced Diana to change her plans to stay with Mr Forstmann in the summer of 1997, saying it was too "dangerous" to take her sons there.

Instead the princess took the fateful decision to take a summer break with Harrods owner Mohamed Fayed. This ultimately led to her going to Paris with his son Dodi, where they died in a car crash.

The revelation from independent inquiries by the Evening Standard comes as it emerged that Princess Diana's phone was bugged by US intelligence agencies on the night she died without the permission of the British secret intelligence services.

Authoritative leaks say the extraordinary revelations will be published this week by Lord Stevens and is bound to raise fresh questions about conspiracy theories.

The US secret service was monitoring Diana's friendship with the controversial financier Mr Forstmann for some weeks.

Mohamed Fayed has always insisted the princess and Dodi Fayed were murdered in a plot involving MI6 agents and US intelligence.

The Standard has learned that Diana had agreed to a week's holiday with princes William and Harry in the US.

She had accepted an invitation from her one-time American boyfriend Mr Forstmann to stay with him at his house in the Hamptons.

But as she was travelling with the princes, she needed the trip to be cleared by the British security services. They surprisingly vetoed Diana's plans because of concerns about the security surrounding the billionaire's homes or perhaps a possible threat from elsewhere.

The decision by the security services ultimately led to Diana striking up her friendship with Dodi and returning to the south of France to holiday with him.

This led to her being in Paris on 31 August, the day of the crash.

The Evening Standard also understands that US secret services have a number of secret files on Diana and her closest associates that are held by the national security agency. The files, which include reports from foreign intelligence - thought to include MI5 and MI6 - come under both top secret and secret categories.The reports cannot be released because of "exceptionally grave damage to the national security". The documents on the princess seem to have arisen because of the company she kept rather than through any attempt to target her.

Diana enjoyed an intimate friendship with Mr Forstmann after her relationship with Prince Charles had broken down.

Lord Stevens is expected to conclude on Thursday that Diana, Dodi Fayed, and their driver Henri Paul died in an accident caused by him driving too fast through the Pont de l'Alma underpass in Paris while under the influence of drink.