Ryanair's outspoken boss Michael O'Leary has hit out at the European Commission which is expected to stop his low-cost airline taking over Ireland's national carrier Aer Lingus.

The attack came amid growing speculation that the ban will be announced by Brussels - the first time for 20 years that the Commission's competition authorities will have blocked a European airline merger.

Mr O'Leary, speaking in Brussels, said there was no justification for the "unprecedented" decision.

He said: "This is designed to appeal solely to the narrow vested interests of the Irish Government and ignore the benefits to millions of consumers."

His £1 billion offer for Aer Lingus immediately attracted a formal Commission monopolies inquiry amid concern about the development of a single dominant air carrier operating at Dublin Airport and stifling competition.

Mr O'Leary said the proposed takeover affected less than 5 per cent of the European airline market.

And he insisted he had already guaranteed massive savings for passengers of about £70 million per year in lower fares and fuel surcharges.

Mr O'Leary declared: "The Commission's decision to prohibit this merger between two EU airlines which between them represent just 5 per cent of European airline traffic is not just unprecedented but, in our view, unlawful.

"We call on the Commission to explain how it can rubber-stamp mergers between larger airlines such as Air France/KLM, Lufthansa/Swiss and Lufthansa/Austrian, when these airlines have bigger positions at their home airports than the combined Ryanair/Aer Lingus share at Dublin Airport."

Mr O'Leary said the Air France/KLM merger had resulted in significant fare increases - but Ryanair has offered unprecedented reductions which, he said, were guaranteed.

He accused the Commission of targeting Ryanair with a "unique" set of rules which did not mirror the Commission's approach to airline takeover cases for the last 20 years.