A second referendum on the UK's membership of the European Union is "not remotely on the cards", Downing Street has said.

Prime Minister David Cameron's official spokeswoman said that last Thursday's vote, which produced a 52%-48% majority for Brexit, was "decisive" and that the Government's focus was on delivering on it.

Her comments came as entrepreneur Sir Richard Branson called on Parliament to take a second look at the EU referendum, warning that the Leave vote has "opened a Pandora's Box of negative consequences" for Britain.

A petition on the parliamentary website calling for a second referendum has attracted more than 3.7 million signatures, though it was revealed on Sunday that nearly 80,000 names have been struck off after being found to be fraudulent.

Asked whether the issue of a re-run referendum was discussed at Cabinet, Mr Cameron's spokeswoman said: "That is not remotely on the cards. There was a decisive result and the focus of the Government is to get on and deliver that."

The Government faces a game of "whack-a-mole" if it wants to prevent people from cheating its online petitions system, a security expert has said.

Authorities were forced to step in at the weekend after it emerged that tens of thousands of names may have been fraudulently added to the petition calling for a second referendum on Britain's EU membership.

The petition has now passed three million signatures, but 77,000 were removed after claims that some of the names were not real.

Commenters on the message board 4chan claimed that they had managed to manipulate the site using "bots" - software applications that can automate web scripts.

Steve Lord, security consultant and researcher at Mandalorian Security Services, told the Press Association that within a matter of minutes he was able to produce a code that allowed him to automatically add multiple signatures to a petition he had created.

"In total the effort required was about five minutes from start to finish, including creating the petition.

"This shows the challenges any online petition site faces. Trying to plug all the holes is as hard as trying to plug holes in a real-life paper petition."

The site now appears to have added additional layers of security, he said, noting that it blocks throwaway domains

He said the ease with which such codes can be produced and adapted makes it difficult for authorities to prevent manipulation.

Mr Lord added: "For the petitions site, they're probably better off setting a policy on what will be accepted, allowing submissions that don't meet that policy through but then silently dropping them."

He said the measure would prevent fraudsters getting feedback from the service to show whether or not their attempt has worked.