Ryanair is to go to court to challenge a Belgian government loan to Sabena.
The European Commission has approved a one-month £80 million emergency bridging loan to save Sabena from bankruptcy.
But Ryanair's chief executive Mr Michael O'Leary claims the cash is an unfair state subsidy which props up a "basket case" company at the expense of more economically viable rivals.
He told BBC Radio 4: "We are going to challenge the loan in the courts because we think this will be the first of a whole series of European airlines which will say 'we have lost money for years, but now Sabena has got some, can we have some too?'
"This is just the latest joke we see emerge from Brussels. Here we have an airline which is probably the greatest basket case in Europe, and has lost money for 39 of the last 40 years.
"Every time they lose money, they get bailed out by the government. It's a joke."
Mr O'Leary said his low-cost airline had opened up seven routes in Belgium and was winning passengers from Sabena because its fares were 80 per cent lower.
And he predicted Sabena would go bust, regardless of the loan.
"In five years' time in Europe, there will be three very large alliances around British Airways, Lufthansa and Air France, and one or two large low-fare alliances - of which Ryanair will be the largest. We might as well get on with it," he said.
But a Sabena spokesman told Radio 4 that the exceptional loan would allow the airline to undergo restructuring into a smaller-scale operation, in keeping with the size of the Belgian market.
PA