Abta chief executive Mark Tanzer welcomed “signs of realism emerging” following an EU-UK agreement on a transition period for Brexit, announced on Monday.

However, Tanzer told an Abta Brexit Briefing in London on Tuesday: “We don’t know where or what we are transitioning to. The next six months represent the biggest political challenge since the Second World War.”

EU and UK negotiators agreed “a large part” of a Brexit transition deal on Monday, adding 21 months to the Brexit process.

The transition will last from Britain’s formal EU exit on March 29, 2019, to the end of December 2020 – assuming the deal is ratified by an EU summit on Friday.

The negotiators also confirmed the post-Brexit rights of EU citizens in the UK – and UK citizens in the EU – and confirmed EU citizens coming to the UK to work during the transition phase would enjoy the same rights.

MP Vicky Ford, chair of the Conservative backbench committee on Brexit, told the Abta briefing: “The deal on transition will hopefully give you that bit of certainty.”

She said: “A fair majority of politicians across Europe want an amicable agreement and a lot face intense pressure from their own voters. The ‘Sunshine Seven’ EU states which are the UK’s main travel markets need to keep travel as easy as possible.”

However, Ford added: “There is a still a huge amount to agree. The issues around the Irish border are complex and very sensitive.”

Tanzer told the briefing: “I’m worried by the ticking clock, [but] encouraged that signs of realism seem to be emerging.

“I’m confident planes will continue to fly between the UK and EU on March 30 next year. But a lot has to be achieved in a very short time to get us there.”

Neil Baylis, partner with law firm K&L Gates, explained: “All EU law will apply in full up to the end of 2020, including the European Court of Justice and [existing] flying rights.”

But he added: “I would caution against undue optimism. Nothing is agreed other than in principle.”

Aviation consultant and former TUI Group head of regulatory affairs Eddie Redfern said: “It’s inconceivable there won’t be an aviation deal – it is in the UK’s and EU’s interests to continue flying.”

UKHospitality chief executive Kate Nicholls agreed, insisting: “It’s inconceivable that we will have planes grounded. It’s inconceivable that politicians would allow it.”

She told the industry audience: “Our biggest concern is about workers. UK hospitality is unique in having a skills shortage and a labour shortage, and 90% of our EU workforce would fail the [government’s] Tier 2 Test [for non-EU labour].

“It’s hugely important we have the [transition deal] commitment that EU workers coming to the UK in the transition period will have the right to remain.”

Nicholls added: “The government’s protections for EU and UK workers have gone a lot further than a lot of businesses realise.”

However, Hotelplan UK chief financial officer Andrew Stewart said: “We send 1,600 staff a year overseas to service ski resorts. I’m very concerned about our niche sector.

“The environment in the EU is much more hostile. We experienced more [employment] inspections in the last two months than in the last five years.”

Nick Parsons, chief editor of online foreign exchange company OFX, pointed out the UK Treasury’s forecasts for economic growth in the next five years “are the lowest in 70 years”.

But Stuart Leven, managing director and vice president of Royal Caribbean International, said: “We may have some challenges moving staff around, we may have some challenges on price, but people will still go on holiday from March next year.”