Two parliamentary committees have referred budget airliner Ryanair to HMRC and the director of labour market enforcement, after the airline refused to cooperate with inquiries into its cabin crew salaries and working conditions.

Frank Field, chair of the work and pensions committee, and Rachel Reeves, chair of the business select committee, have urged tax and employment authorities to investigate the Irish carrier and the agencies that supply cabin crew to the airline.

"We believe it is vital that potential poor employment practices are examined not only to ensure the rights of Ryanair cabin crew are protected, but also to ensure that there is no 'race to the bottom' across the aviation sector," Field and Reeves wrote.

The former added: "We and the public can draw our own conclusions about Ryanair's comprehensive failure to answer allegations on its pay and employment practices."

Towards the end of 2017, MPs asked Ryanair to shed light on its pay and employment practices, after it emerged cabin crew were required to work for free, pay for their own uniforms and take long period of unpaid leave.

However, the airline's initial response was deemed unsatisfactory and the Dublin-based carrier subsequently declined to answer further questions.

A letter from Ryanair's HR director, sent on 21 December, said crew earned between €24,000 (£21,150) and €40,000 a year, double the legal minimum for the work carried out. However, the committees said the figures did not match what they had seen on a contract.

"Ryanair might want to dodge our committees' questions and hide behind excuses, but the UK is the largest provider of workers to the company and we must be sure that everyone working for Ryanair is receiving fair pay and reasonable working conditions," said Reeves.

The airline has said pilots across its 15 bases in Britain had agreed to accept a salary increase of up to 20%, while negotiations continued with British pilot union Balpa over industrial union recognition.

"Ryanair has already confirmed to this Committee that its pilots earn between €130,000 and €180,000 p.a. and our cabin crew between €24,000 and €40,000 p.a. which is more than double UK National Minimum Wage and since we also comply fully with UK employment law, we have no further comment in response to this Committee's inaccurate press releases," the airline said in a statement.

In December, the Irish carrier in a climbdown said it was prepared to recognise pilot unions, as it sought to avoid a series of strikes that would have wreaked havoc on its Christmas schedule.

The move marked a major milestone for the airline, which had steadfastly refused to recognise unions, and came after pilots in the UK, Germany, Italy, Spain and Portugal had threatened walkouts.

Michael O'Leary, the airline's chief executive, said the decision to recognise unions had been taken to ensure flights over the Christmas period would go ahead as planned.

"We have written to these unions inviting them to talks to recognise them and calling on them to cancel the threatened industrial action planned for Christmas week," he added.

"Recognising unions will be a significant change for Ryanair, but we have delivered radical change before."