Patino stood trial last year alongside other members of a gang who were convicted of smuggling copies of Viagra and medicines to treat baldness from illicit factories in China, Pakistan and Asia.

The jury could not decide on verdicts on Patino and he was due to appear for a retrial this week but today entered a guilty plea to a charge of conspiring to sell counterfeit Viagra at Kingston Crown Court.

He will be sentenced next week.

The court was told Patino was in league with the lynchpin of the operation Ashish Halai, 34, of Borehamwood, Hertfordshire, who was jailed for four-and-a-half years last year.

Instead of the medicine coming in the manufacturer Pfizer's packaging, they arrived to customers in brown bottles.

The labelling gave a return address as a Finnish post box number to deliberately conceal the identities of those behind the operation.

The court was read a number of emails from disgruntled customers complaining that the tablets were fake and had no effect.

Patino deposited over £1.3 million into a Singapore bank account in the name of Halai.

He pleaded guilty on the basis that his criminality netted him $237,000 (£133,000).

Patino, who qualified as a doctor in 1994, is a Mexican national with a US passport.

He was arrested by investigators from the Medicines and Healthcare products Regulatory Agency (MHRA) who had been tracking the gang when he arrived at Heathrow Airport in October 2005.

He had been due to catch a connecting flight to Leeds for a business meeting in the Midlands.

Investigators said the huge amount of fake tablets he was peddling earned Patino the title the "King of Viagra" among his associates.

He had a previous conviction in 1998 for smuggling a human growth hormone into the US, the court heard.

Last year Salesman Gary Haywood, 60, from Leicester was jailed for six years; student Ashwin Patel, 26, of north London given 18 months and businessman Zahid Mirza, 48, of Ilford, Essex sentenced to two and half years for being part of the conspiracy.

The supply ring sold tens of thousands of tablets in the UK, America, the Bahamas and Mexico. It involved scores of businesses, both real and fake.

The drugs were bought from Chinese suppliers for as little as 50 US cents (25p) but sold on to unsuspecting customers for up to £20 per tablet.

The case was the largest of its kind to be heard in the UK.

Judge Nicholas Price QC released Patino on bail until next Tuesday when he will be sentenced.

He told him: "In my view there is only one appropriate sentence and that is one of an immediate period of custody.

"You must understand that I grant you bail in those circumstances.

"When you come to court on Tuesday you will be sentenced to a term of imprisonment."

Mick Deats, head of enforcement at the MHRA, said: "Dr Patel is the last in a group of businessmen engaged in this conspiracy.

"Counterfeit medicines are dangerous. They are designed to deceive patients and health care professionals and generate vast profits."