Aer Lingus suffered a €12 million increase in losses to €16.4 million in the first half of the year.

The Irish carrier attributed the higher operating loss for the six months to June 30 from €4.4 million a year earlier to one-off factors such as contracted flying start-up costs to operate Virgin Little Red UK short-haul services, planned changes to its long-haul fleet, maintenance costs, foreign exchange and weaker trading on UK routes.

Bookings for the second half of the year are ahead of the same period in 2012.

But the carrier admitted that the warm weather “negatively impacted” bookings in July.

“We maintain our guidance that 2013 operating profit, before net exceptional items, will be broadly in line with 2012,” the airline said.

Passenger carryings rose by just 1.3% to 4.57 million in first the six months of the year.

The wet lease agreement with Virgin to operate flights from Heathrow to Manchester, Aberdeen and Edinburgh is expected to “broadly break even” this year after significant start-up costs before becoming profitable in 2014.

A final report from the UK Competition Commission into its probe into Ryanair’s minority shareholding in Aer Lingus is expected in August.

Aer Lingus chief executive Christoph Mueller said: “Our Q2 2013 revenue performance was particularly strong. We expanded long-haul capacity by 16.3% in the quarter and successfully sold the additional seats, achieving a load factor of almost 95% in June.

“Short-haul continues to trade positively. However, the weakness in UK routes identified in our Q1 results has continued in Q2.

“The first half of our financial year is seasonally loss making and we are reporting an operating loss (before exceptional items) which is €12 million higher than the prior year.

“This performance reflects the impact of a number of one-off factors including the start up of our contract flying operations and planned changes to our long haul fleet.”

He added: “We continue to focus on our cost base and are conscious that certain planned cost saving initiatives have not had effect as quickly as we had initially hoped.

“However, the voluntary severance programme we outlined at Q1 seeking a headcount reduction of 100 has been oversubscribed with expressions of interest. We expect the benefits of this programme will start to take effect towards the end of the current year with full year effect in 2014.”