Nigella Lawson has been stopped from flying to the US four months after she confessed to taking cocaine.

The TV cook, 54, was barred from boarding a flight from Heathrow to Los Angeles at the weekend.

At the of trial two personal assistants employed by Lawson and her ex-husband Charles Saatchi the self-styled "domestic goddess" admitted taking the class-A drug.

Lawson, a mother of two, told jurors that she took cocaine with her late husband, John Diamond, when he found out he had terminal cancer and in 2010 when she claimed she was being "subjected to intimate terrorism" by Saatchi.

A witness told the Daily Mail that Lawson attempted to board a British Airways flight from London on Sunday morning but was turned away.

"She didn't seem to say much but she did not look happy," the onlooker said. "She could not get on the flight so she had to turn around and leave."

Police reviewed the allegations of drug use made during the trial but Scotland Yard said no further action would be taken.

While Lawson has not been convicted for any drug offence, it appears that US officials have still decided to deny her entrance to the country.

The US department of homeland security told the Mail that foreigners who had admitted drug taking were deemed "inadmissible".

However, Lawson has visited America since the court appearance. She appeared on the US TV show Good Morning America to promote her show, The Taste, but also spoke at length about the trial of Elisabetta and Francesca Grillo, who were acquitted.

The Grillos had been accused of fraudulently using company credit cards, spending hundreds of thousands of pounds on designer goods for themselves while working as personal assistants to the celebrity couple. But the Italian sisters claimed every purchase had been approved by their then bosses and they were found not guilty after jurors deliberated for nine hours.

Lawson told Good Morning America that having details of her acrimonious split from the millionaire art dealer Saatchi discussed in court under the glare of the world's media had been "mortifying".

Lawyer Susan McFadden, who specialises in US immigration law, said Lawson was unlucky because her fame had drawn attention to the comments in court about drugs.

She explained: "In order to travel to the States without a visa on the visa waiver programme one has to obtain Esta authorisation [electronic system for travel authorisation].

"As part of that one has to answer questions about whether one has been arrested or convicted for an offence involving moral turpitude or in relation to a controlled substance. Someone in Ms Lawson's circumstances could tick no.

"But the problem comes when one also has to answer a question as to whether you are a drug abuser or addict.

"The question of who is a drug user is a difficult one and is supposed to be directed by regulations set out by the Centre for Disease Control and Prevention in Atlanta.

"Typically any person who has used drugs within the last year can be considered a drug abuser or addict.

"It could be because she is unlucky enough to be famous and her admission of drug use made the newspapers. It could be that someone said: 'Hey, if she has used illegal drugs in the past year she could be classed as a drug abuser'."

McFadden explained that if this was the case, there were a series of steps which must be taken before Lawson would be allowed back into the States.

She would first be advised to visit a doctor who holds a contract with the US embassy in London who would carry out an assessment to see whether or not she could be classed under these terms.

If the doctor says she should be free to travel she could obtain a visa in a number of weeks, McFadden said. Lawson said in court that she last used cocaine in 2010.

She told the court: "I have never been a drug addict. I've never been a habitual user. There are two times in my life when I have used cocaine."

A spokeswoman for the US embassy said: "There are several ways of legally travelling into the United States and Ms Lawson has been invited to come to the embassy and apply for a visa for travel to the US. We understand she has professional requirements for US travel and these matters are generally handled routinely and expeditiously, so stand by."