A leading plastic surgeon who was threatened with libel action after questioning the claims made for a breast enhancement cream has been vindicated after a watchdog found no evidence that the product increased bust size.

Dr Dalia Neild of The London Clinic said it was "highly unlikely" the £125 cream, named Boob Job, could increase a woman's bust size by 2.5cm without the need for surgery, as advertised.

Manufacturers Rodial said the effect could be achieved if women rubbed the cream into their breasts for 56 days. Users were said to include Scarlett Johansson and Victoria Beckham. Dr Neild, who also questioned the safety of the treatment, was threatened with legal action by Rodial after her comments were quoted in a newspaper.

Dr Neild, the latest in a line of scientific figures to be threatened with legal action after speaking out on matters of public health, said she had a "duty to speak out when products making these claims are not backed up by evidence".

Yesterday the Advertising Standards Authority ruled that claims that the cream significantly increased the volume and firmness of breasts were misleading as there was no evidence to back them up.

It found Rodial had made "unauthorised medicinal claims" by advertising a gel called Volufiline as a major ingredient. Rodial said the gel "promotes lipid storage leading to an increase of adipocyte volume in the fatty tissue" causing "up to 8.4 per cent improvement in volume".

The watchdog said the Medicines and Healthcare products Regulatory Agency had confirmed that Volufiline was not licensed for use in the UK. The claim to improve breast size by 8 per cent "implied the modification of human physiology by way of pharmacological and/or metabolic pathways" which was not justifiable.

It also considered testimonials such as "I've gone from a 32A/B to a much fuller and firmer 32C", and "I am now a fuller, firmer and happier 36D". The watchdog said: "We had not seen any documentary evidence in support of those claims and therefore concluded that the ad was likely to mislead."

Rodial was banned from repeating the claims, which had feature on its website. A colleague of Dr Neild said: "She is very pleased with the ruling. She thinks it's important for medical and scientific figures to speak out but this has been a difficult time for her."

The ruling follows the case of Dr Simon Singh, the author and broadcaster who was sued by the British Chiropractic Association after criticising the lack of evidence for the effectiveness of the method in treating certain child disorders.

Scientists and freedom of speech campaigners argued that Dr Singh had a right to express his opinion in print. The BCA dropped the case.

Robert Dougans, Ms Nield's solicitor, who also acted for Dr Singh, said: "This vindicates Dr Neild's courage in speaking out about this product. Unless the libel laws are drastically reformed, other doctors and scientists are going to be forced to spend more time with lawyers."

The charity Sense about Science is campaigning for a change to English libel law to protect debate about science and medicine.