The warnings will begin to appear in spring next year if the government has failed to break the deadlock in the ongoing Brexit negotiations.
It would affect tickets to EU destinations as well as up to 17 other countries, including the US, as flights to places like America are covered under current EU regulation.
If flights are grounded as a result of ‘no deal’, then airlines would compensate passengers for the cost of their flights.
But any extra costs usually covered by airlines based on EU regulations – like compensation – would not be paid out.
Air traffic in the EU is currently governed by the Single European Sky arrangement that allows planes to fly without restriction from one European country to another.
For things to continue as such, a new treaty will have to be agreed between the UK and the EU.
It comes a month after UK Chancellor Philip Hammond warned MPs that all air traffic could ‘theoretically’ stop the day after Brexit.
He said today during a select committee: “It is theoretically conceivable that in a no-deal scenario there will be no air traffic moving between the UK and European Union on March 29, 2019 – but I don’t think anyone seriously believes that is where we’ll get to.
“So there are a range of outcomes. What we will need to do at a point in time is determine what a realistic worst case scenario that we need to plan for and invest for.”
Airlines reportedly continue to be optimistic that a deal will be struck between Britain and the EU.
But there are concerns the government won’t approve an aviation deal if it has to abide by certain European Courts of Justice rulings.