An out vote will have wider-ranging ramifications for the trade – with issues such as open skies agreements and the package travel directive potentially under threat – but the problem voiced by a number of firms contacted by TTG is that it is impossible to plan for the unknown.

Some companies have also taken the step of articulating their views on the vote to their staff – while stressing that at the end of the day it remains the employee’s personal decision.

Tui UK and Ireland has informed workers of its remain position, insisting that British holidaymakers benefit from being part of a common European market.

A spokesperson told TTG: “It’s too early to speculate on any conclusions or actions that may or may not come out of the final vote. Ultimately the decision lies solely with the British public, [but] we have contingency plans in place that take into consideration possible scenarios after the referendum.”

Other UK-based firms are also exploring their options. Low-cost carrier easyJet is reportedly looking into setting up a separate European unit. The Luton-based airline told TTG an exit from the EU “could have implications”.

Andrew Swaffield, chief executive of the Monarch Group, has been forthright in expressing his opinions. Earlier this year he appeared in a TTG debate on Brexit, arguing that on balance the UK was better off in. He has also articulated this position to staff and encouraged them to write to him if they have any questions.

The company said it was unable to plan for a potential Brexit until it knew the shape of the UK’s new relationship with the EU.

A spokesperson added: “As Monarch has no EU bases and 85% of our customers are UK citizens, we are less affected than some other airlines.

“Clearly we will be affected by changes in consumer confidence, exchange rates and other short-term shocks to the economy that we believe may follow on from a leave vote.

“This is the kind of normal contingency planning that we always carry out when thinking about our financial performance. Longer term changes to air fares and competition issues will take more time to analyse.”

Thomas Cook has stressed that although it believes in the “free movement of goods and services”, the decision is “a matter for the British people to decide”.

“Our job is to ensure that we are prepared for whatever the outcome of that decision will be,” the company said.

Meanwhile, founder of Virgin Atlantic Sir Richard Branson this week launched his own campaign to help try and keep Britain in the EU.

A spokesperson for the airline said: “The EU deserves credit for its overarching agenda to liberalise aviation, which has helped deliver stronger competition, and ultimately benefits consumers.

“Overall, we are supportive of remaining in the European Union, but continue to urge reforms that will improve the entire travel sector.”