The UK aviation industry is 365 days away from needing to know how it will be affected by the Brexit negotiations, Ryanair has claimed.
The low-cost carrier, which is the country's second largest airline by passenger numbers, warned that it could make "significant capacity cuts" unless there is more "clarity" in the next 12 months.
UK ministers have previously said maintaining liberal access to European aviation markets is a "top priority".
The single market for aviation, created in the 1990s, means there are no commercial restrictions for airlines flying within the EU.
Ryanair's chief marketing officer Kenny Jacobs said: "We all respect the intent of the Government, that they want to remain in open skies, that they want to work out a good agreement and that they want to stay close to Europe.
"But we don't see a plan yet, and the clock is ticking.
"We are 365 days away from needing to know what is going to happen."
Prime Minister Theresa May has vowed to trigger Article 50 by the end of March, beginning the two-year process of leaving the EU.
Mr Jacobs accused the Government of being "a bit slow getting their own plan in order", saying a number of pro-Brexit ministers "had a plan to win the referendum" rather than what to do next.
He told a press conference in London that airlines "plan a year in advance" and therefore "need to know how is this going to work" by the end of February or the start of March 2018.
"This isn't about having a new arrangement in place for the summer of 2019," he said.
"This is about having clarity on what are the options and what way is this going to work legally, technically, regulatory, this time next year.
"If we don't know this time next year then what are airlines going to do? That's when we might have to say 'are we going to have to make significant capacity cuts?'"
He added: "I hope this doesn't happen."
Ryanair previously announced it will "pivot" growth away from UK airports and focus more on growing airports in the rest of Europe following the Brexit vote.