Qantas will axe its direct flights between Adelaide and Singapore in April as part of a national restructure of its Asian services.
Adelaide Airport says the airline provides three weekly flights direct from Singapore to Adelaide and another three from Adelaide to Singapore via Sydney.
The airport says the decision is disappointing and comes as the number of international passengers using the terminal continues to grow.
Adelaide Airport says the loss of the Qantas flights will be partially offset by Singapore Airlines' decision to increase its Adelaide services in July.
But Senator Nick Xenophon has labelled Qantas's decision an insult to the state.
He says Adelaide is being marginalised by the airline.
"We now have a crazy situation where other countries' airlines are bringing more and more flights into Adelaide from overseas yet Qantas has now severed its last remaining flights in and out of Adelaide on an international route," he said.
"That, to me, is a disgrace.
"Qantas says that they've been losing money on the Adelaide-Singapore route when Qantas employees tell me that they're flights out of Adelaide to Singapore have invariably been incredibly full." The Transport Workers Union says it expects the airline's decision will lead to job losses.
Branch organiser Matthew Spring says Qantas workers were given no warning about the changes.
"They only found out in the same that way we did, through the media and through us ringing them.
They're shocked," he said.
"One of my delegates said 'I don't understand that, those flights are coming in loaded, they're not empty, they're coming in fully loaded.
Why would you want to get on a domestic flight and fly to Melbourne to fly out to Singapore, when you go straight from here to Singapore on another flight?'" Qantas is forming an alliance with Dubai-based airline Emirates.