The massive outlay on charter aircraft came as the Government’s Learjet has been dogged by reliability issues and has not been tasked with a single mission this year.

Figures from the Department of Defence show €617,700 was spent on private jets last year with further expenditure of €269,461 in the first four months of this year.

The most expensive flight came with a €158,000 bill as it took Tánaiste Micheál Martin from Dublin to Cairo and Tel Aviv last November for talks over Irish citizens in Gaza.

However, that bill may yet be outdone by another charter flight from April when Mr Martin flew to Cairo, El Arish, and Amman as the department are still “awaiting [an] invoice” for that trip.

The Department of Defence said that private planes had been leased on 15 separate occasions since the beginning of last year, all for trips involving either the Taoiseach or the Tánaiste.

The expenditure began last May when a jet was hired at a cost of €72,000 to bring then-taoiseach Leo ­Varadkar to Chisinau in ­Moldova on an overnight trip for a meeting of EU leaders.

In October, an aircraft was again chartered to take Mr Varadkar to Brussels, also for a single night, with the bill coming to €48,000.

A sum of €24,700 was spent on a jet to bring the former Taoiseach to Copenhagen in November last year before a €46,249 trip to Brussels in December of last year.

The bill for Mr Varadkar’s trip to Pristina, Podgorica, and Skopje in the Balkans in January this year came to €108,864 according to details released by the Department of Defence under Freedom of Information laws.

A further €59,108 was spent in late January for an overnight trip to Zurich for the Taoiseach and €45,674 for another journey to Brussels later that month.

Further bills included €70,743 for a trip to Munich in February, €97,383 for a charter to Brussels in March, and €87,576 for a jet to fly to Brussels and Warsaw in April of this year.

Costs for the trips varied widely depending on what aircraft was used, with higher per hour bills when a larger delegation needed to travel abroad on official duties.

In addition, some of the trips were paid for through the purchase of “pre-paid flight hours” meaning costs incurred last year may have been used for journeys that took place this year.

The use of charter aircraft has become a regular cost because the Government’s Learjet that would normally be used is no longer considered reliable.

On at least three occasions, senior politicians have ended up “stranded” at airports abroad after the jet developed technical issues during ministerial air transport missions.

The situation came to a head last December when Simon Harris, who was then Further and Higher Education Minister, arrived at Baldonnel Airport and boarded the Learjet, only to be told the €8m aircraft had suffered yet another fault, causing him to miss an important meeting in Belgium.​

In emails written in the aftermath of the fiasco, the secretary general of the ­Department of Defence ­Jacqui McCrum said that “all confidence in the Lear is gone” and the aeroplane has not been used this year.

Instead, the department has been hiring charters for the Taoiseach and Tánaiste and on occasion using a smaller PC-12 aircraft for travel.

The PC-12 has been tasked with four missions since the beginning of this year, with flights to Brussels, Munich, Luxembourg, and London, according to a log published to the department’s website.

The Government has already gone out to tender for a new €45m aircraft to replace the ailing Learjet, but military sources said that process was likely to be lengthy.

In tender documents, the Department of Defence has said they are looking for an aircraft with no less than 10 seats, a range of more than 4,500 miles for trans-Atlantic travel, and the capability of being retrofitted with defensive aids if that was ever required.

Asked about the costs incurred on leasing private planes, the Department of Defence said they had no further comment to make.