The Hong Kong-based airline Cathay Pacific is selling seats from Sydney to London for more than £20,000 in April, as passengers search for scarce long-haul flights without changing in the Middle East.

The tickets, listed at A$39,577 in business class for returns departing in mid-April, far outstrip the usual fares charged even in the first class cabin.

Travellers on routes around the world have had to look for different options after the US-Israel war on Iran closed critical airspace and major hub airports in the Middle East.

A large proportion of UK-Australia traffic connects in the Gulf, travelling on airlines including Dubai-based Emirates, Abu Dhabi’s Etihad and Qatar Airways out of Doha. The three biggest carriers have now restarted limited operations at a fraction of normal schedules, and hundreds of thousands of travellers have had their flights cancelled over 10 days of full or partial airspace closure in countries around Iran.

Alternative long-haul connections, such as via Singapore, Hong Kong or Malaysia, have limited capacity.

Economy flights via the Gulf in the same period remain on sale from around £1,100 return, the top end of the normal range according to Skyscanner, although bookings are mired in uncertainty as the war continues. Routes on carriers operating via China, India and Malaysia typically were listed at £1,400-£1,800.

While April is a busier season, return UK-Australia fares on the route are more typically £3,000-£4,000 in business class and from £800 in economy.

Cathay no longer had seats in economy class available to book on many dates in April but fares for the cheap seats started at around £1,800.

Any passenger that now stumps up the A$39,577 fare will be travelling in mixed cabins – upgraded to first for some legs, but on certain departures, with the inconvenience of a stint in economy for an Australian short hop. The extraordinary fare exceeds the listed price for first class, available for a mere A$28,146 (£14,900) in April.

Andrew Charlton, an aviation analyst, said he had not seen fares at a similar level but was aware of other airlines in Asia ramping up fares on a variety of routes to Europe, including from the Philippines and Japan, with many fares in economy on the popular UK-Australia route now at last year’s premium economy levels.

The closure of Russian airspace has made the Asian routes less viable, with carriers either having to divert south over the same congested and uncertain Middle East airspace, or even from Japan, rerouting north-east over Alaska and the Arctic to reach London.

Long-haul prices advertised via the Gulf remain cheapest, but Charlton said: “I wouldn’t do it without checking my travel insurance. The way Emirates is going to attract passengers back is by selling cheap – but you would be taking a risk.”

Cathay was approached for comment.