Police are investigating after Tube passengers reported being targeted at London Underground station ticket machines by scamming devices.

Passengers have been vocal on social media about potentially being the victims of so-called “skimming” operations at Tube stations.

Gangs of thieves have been known to attach devices to ticketing machines that copy the user's card details as they buy Tube tickets or top up their Oyster cards.

Some people have reported having money taken from their bank accounts after using one of the machines targeted by scammers.

British Transport Police have arrested a number of individuals in connection with Underground station tampering, according to the Metro.

A Reddit user earlier this month warned Londoners that they believed they had been a victim of skimming fraud at Piccadilly Circus Underground station, served by the Piccadilly and Bakerloo lines.

“There was a very discreet skimmer attached to the ticket machine when I tried to top up my Oyster card, it tried to take £250 as an ATM withdrawal from my card,” the user said.

“They also somehow deactivated the contactless function which made me have to insert my card. The Transport for London staff said it was the third time this has happened! They are getting very smart with the discreet devices they are using.

“Just be careful guys and, where you can, use Apple Pay and, if it doesn't work for some reason, do not insert your bank card!”

Another Reddit user reported a similar scam at St Paul’s Underground station, which is served by the Central line.

They said the ticket machine would not accept contactless payment or payment by PIN. They said there was later a £500 withdrawal from a cash machine in the area.

“I never noticed anything dodgy with the machine, [so] be careful topping up,” the user said.

“Almost got stuck in central London with an empty Oyster card and blocked bank card.”

They said that, after reporting the incident to British Transport Police and Transport for London, their bank refunded the money.

DI Marvin Bruno, in comments provided to the Metro, said: “We’re leading an investigation following recent reports of fraudulent transactions on some ticket machines in London Underground stations.

“We have already arrested people in connection, and our enquiries into the fraudulent activity are continuing at pace.

“We continue to work closely with Transport for London and partner agencies to identify those responsible and prevent further offending.”

TfL is assisting police with their investigations, the news outlet reported.

The newspaper said a French businessman had more than £1,500 stolen after using an affected machine at Piccadilly Circus.

Etienne Bouvier, who had been visiting London for work, spotted that two cash withdrawals of £800 had been made at an ATM after he had left the capital.

He said the two fraudulent transactions were recorded as happening in the City of London. Mr Bouvier, from Lille, is reportedly struggling to have the stolen cash refunded.

The Standard contacted British Transport Police for comment.