The looming return of roaming charges after Brexit will cost business people visiting the EU up to £778 a month, new research shows.

The fees for using international data within the EU – axed under a Brussels agreement last June – are set to be brought back after Theresa May announced the UK would leave the “digital single market”.

Now the anti-Brexit Best for Britain group has calculated the financial hit for business travellers, if prices return to the level before roaming charges were abolished.

The research puts the extra bill at £195 if foreign mobile companies exploit their new freedom to ramp up the price for local firms to use their networks.

But the additional charges soar to £778 if those local mobile carriers also push up the cost for their customers to the maximum allowed before the cap.

Best for Britain said it showed “the reality of the decision to leave the European Union is dawning”.

The expected fees are much higher than the £61 top-up run up by holidaymakers before the EU acted – because business travellers use so much more data.

They consume 4.5 gigabytes (GB) – for a typical six days abroad each month – sending many hundreds of emails containing graphics, on video conferences and sharing work on social media, perhaps including videos and music.

“The cost of a hard Brexit on British travellers is becoming abundantly clear,” said Alex Sobel, a Labour MP and supporter of the anti-Brexit Best for Britain group.

Brexit bonhomie as May finally seals agreement with EU leaders Show all 10 1 /10 Brexit bonhomie as May finally seals agreement with EU leaders Brexit bonhomie as May finally seals agreement with EU leaders Britain's Prime Minister Theresa May is welcomed by European Commission President Jean-Claude Juncker Reuters Brexit bonhomie as May finally seals agreement with EU leaders EU President Jean-Claude Juncker greeting Theresa May at the EU Commission in Brussels PA Brexit bonhomie as May finally seals agreement with EU leaders Britain's Prime Minister Theresa May poses for a picture with European Council President Donald Tusk REUTERS Brexit bonhomie as May finally seals agreement with EU leaders EU Commission President Jean-Claude Juncker walks behind British Prime Minister Theresa May EPA Brexit bonhomie as May finally seals agreement with EU leaders British Prime Minister Theresa May (L) and European Commission President Jean-Claude Juncker address a press conference at the European Commission in Brussels AFP/Getty Images Brexit bonhomie as May finally seals agreement with EU leaders British Prime Minister Theresa May speaks at a press conference with EU Commission President Jean-Claude Juncker EPA Brexit bonhomie as May finally seals agreement with EU leaders Britain's Secretary of State for Exiting the European Union, David Davis (L) and Michel Barnier (2-L), the European Chief Negotiator of the Task Force for the Preparation and Conduct of the Negotiations with the United Kingdom under Article 50 enter the room by the emergency exit to attend British Prime Minister Theresa May press briefing on Brexit Negotiations in Brussels. EPA Brexit bonhomie as May finally seals agreement with EU leaders Britain's Secretary of State for Exiting the European Union David Davis (L), Britain's Prime Minister Theresa May (2-L), European Commission President Jean-Claude Juncker (2-R) and European Union's chief Brexit negotiator Michel Barnier (R) in a meeting on Friday morning AFP/Getty Images Brexit bonhomie as May finally seals agreement with EU leaders European Union chief Brexit negotiator Michel Barnier speaks during a media conference at EU headquarters Brexit bonhomie as May finally seals agreement with EU leaders European Council President Donald Tusk addresses a media conference at the Europa building in Brussels AP

“Not only will they suffer longer queues as hard borders take effect, but they will be hit in the pocket with the return to the roaming charges that were recently outlawed by the EU.

“British businesses large and small will be put at a disadvantage as they are forced to pay big roaming bills whilst our nearest neighbours pay nothing.”

And Mike Spicer, director of research at the British Chambers of Commerce, said: “Businesses want answers to the practical questions they face when conducting business in Europe.

“They will want to see the UK and the EU reach a deal to ensure that roaming charges don’t come back, which would add unnecessary cost to doing business overseas.”

The Government has admitted it will need to strike a special deal with the EU in order to avoid a return of roaming charges after Brexit.

It is widely assumed that will be difficult outside the digital single market, whose purpose is to eliminate digital regulatory barriers between EU member states, including phone charges.

A committee of MPs which has studied EU regulations in detail has argued it will be impossible for mobile operators to avoid reimposing fees, as their operating costs rise sharply.

Furthermore, EU commissioner Günther Oettinger, told the European Parliament that a standalone UK-EU roaming agreement would be in breach of World Trade Organisation rules.

The Prime Minister is aiming to strike a wide-ranging free trade agreement with the EU after Brexit – but no such agreement to date has incorporated rules on roaming charges.

The Department for Culture, Media and Sport said two phone operators – Vodafone and Three – had already committed publicly not to impose roaming charges after Brexit.

But it admitted: “Arrangements on mobile roaming would be subject to any negotiations. However, a future partnership between the UK and EU is clearly in the interests of both sides.”

Ms May revealed the exit from the digital single market in last month’s Mansion House speech, in which she said Britain had to face up to “hard facts” about Brexit, including reduced single market access.

“On digital, the UK will not be part of the EU’s digital single market, which will continue to develop after our withdrawal from the EU,” she said.

“This is a fast evolving, innovative sector, in which the UK is a world leader. So, it will be particularly important to have domestic flexibility, to ensure the regulatory environment can always respond nimbly and ambitiously to new developments.”

It is unclear whether fees will return as early as Brexit day next March, or at the end of the planned 21-month transition period – given the UK will effectively remain within the single market in the interim.