Ryanair chief executive Michael O'Leary yesterday conceded that his decision to spend €390m of shareholders' money on a stake in Aer Lingus was "stupid".

The admission came at a wide-ranging media briefing, where Mr O'Leary called on the Government to axe the €5bn metro, the aviation regulator's office and the €10 aviation tax.

Speaking in Dublin, Mr O'Leary also insisted he was "completely serious" about charging for onboard toilets and had already instructed plane-maker Boeing to examine installing a credit card swipe machine on toilet doors.

Mr O'Leary later confirmed he had written to the Transport Minister to express his outrage at the Aviation Regulator's recent decision to ignore a government-commissioned report that said Terminal 2 "may be greatly oversized".

The Ryanair boss's comments on the wisdom of his Aer Lingus investment came after he told journalists that he was "very unlikely" to make another bid for the rival airline and would write his 29pc stake "down to zero" if accounting rules allowed it.

"It was a stupid investment," he added. "At the time, it was the right strategy to go for one combined airline but it [the investment] has now proven to be a disaster."

He added that he'd told shareholders he'd "f***** up" but hadn't apologised.

"I never say sorry, except to my wife who I apologise to all the time," he said.

The briefing was called to unveil Ryanair's tourism strategy, which included the abolition of the metro, the aviation regulator's office and the €10 aviation tax.

Mr O'Leary also called for the abandonment of state subsidies for regional air routes, including the subventions for Ryanair's Dublin/Kerry service, and for a 30pc reduction in charges at Dublin, Cork and Shannon airports.

The Ryanair boss said he had outlined the plans to the Government's Tourism Renewal Group earlier in the day.

Mr O'Leary also told journalists he'd been in talks with Boeing on charging for toilets for several years but the plane-maker couldn't design a slot to accept coins. "We're now looking at credit cards," he added.

Those who won't pay and soil planes will be "fined €100", he said.