Britain and Canada have advised travellers not to use Iran's main new international airport because of concern over its safety, just hours before the airport was due to receive its first flights.
"We are aware of reports that the runway at the new Imam Khomeini International Airport in Tehran may not yet be suitable for use," the British Foreign Office said of the airport, due to open on Saturday.
"We are in contact with the Iranian authorities about this. Until the situation has been clarified, we advise British travellers to travel by flights using the existing (Mehrabad) airport."
The Canadian government issued a virtually identical warning. It was not immediately clear whether any other countries were giving the same advice.
Iran said the runway had been approved by international experts.
"I don't know the reason behind this advice," said Reza Jafarzadeh, spokesman for Iran's Civil Aviation Organisation.
"The runway is ready and this has been approved by ICAO (International Civil Aviation Organisation), otherwise the organisation wouldn't have issued permits for today's flights," he said.
The opening of the Imam Khomeini International Airport, built to serve the capital Tehran, has already been aborted once, when military vehicles swarmed onto the runway and closed it down a year ago after an inaugural flight had landed.
The military had expressed concern about security as the airport was being run by a Turkish company. After a year of wrangling the airport will now be operated by Iranian carriers, led by flagship airline Iran Air.
Flights to and from the airport, 45 km south of Tehran and designed to replace the ageing Mehrabad International Airport close to the city centre, were due to resume on Saturday afternoon with carriers serving the nearby United Arab Emirates.
Flights from other Gulf destinations will be added from May 9 and all international flights will use the new airport by March 2006, Jafarzadeh said.
Neither Britain or Canada said exactly why they were concerned about the safety of the runway.
But an Iranian aviation source has told Reuters buildings at the new site have suffered from subsidence and there were fears the runway could not cope with the weight of a heavy aircraft landing on the tarmac.
Some European carriers, expected to switch flights to the new airport in coming months, have expressed concern about the lack of public transport links to the new site, which, because of Tehran's traffic congestion, can take more than 90 minutes to reach from parts of the capital.