The CEO of Irish low-cost airline Ryanair has attacked the UK government-backed rescue of rival airline Flybe, joining industry heavyweight Willie Walsh in opposing the state aid for the private company. 

Michael O'Leary said on Thursday, he had written to UK finance minister Sajid Javid, calling on him to extend the same benefits to other airlines that he had granted to the beleaguered Flybe. O'Leary intends to launch legal proceedings against the government if they refuse. 

"This government bailout of the billionaire-owned Flybe is in breach of both competition and state aid laws," he said in a statement, adding that any dispensation Flybe had been given on air passenger duty (APD) should be extended across the industry. 

Flybe was saved on Tuesday after its shareholders agreed to invest more money alongside a UK government support plan. Details of that support have not been made public, though it is believed to include plan to review the £13 ($16) APD on domestic flights in the government's March budget.  

Meanwhile, the Stobart Group, one of Flybe's three shareholders, announced on Thursday that under the agreement with the government it would invest 9 million pounds ($11.77 million) in the company. 

Flybe is owned by Connect Airways, a consortium created by the Richard Branson founded-Virgin Atlantic, the Stobart Group and investment adviser Cyrus Capital. 

Walsh, CEO of IAG, the parent company of British Airways, has echoed O'Leary's complaints about the rescue, filing a complaint with the EU, saying that help for Flybe constituted a breach of state aid rules. 

The British government has defended the rescue deal and said it is fully compliant with state aid rules. 

The government's decision also irked environmental campaigners. UK Green Party MP Caroline Lucas, tweeted that the idea of reducing air passenger duty was "utterly inconsistent with any serious commitment to tackle #climatecrisis." She added on Twitter: "Domestic flights need to be reduced, not made cheaper." 

The government's move could save several thousand jobs and was hailed by the general secretary of the British Airline Pilots Association, Brian Strutton, a trade union. He said: "This is good news for 2,400 Flybe staff whose jobs are secured and regional communities who would have lost their air connectivity without Flybe."