The engineering arm of Monarch Airlines is this week scrambling to put together a financial restructuring deal amid the threat of a possible winding-up petition from the taxman.
Sky News has learnt that Her Majesty's Revenue & Customs (HMRC) has drawn up plans to take action against Monarch Aircraft Engineering (MAEL) over an unpaid tax bill, the size of which was unclear on Thursday evening.
The move raises questions over the future of a business which employs more than 800 people and provides critical services to Britain's aviation sector, working with airlines including Cathay Pacific, easyJet, Norwegian, Virgin Atlantic and Wizz.
It comes almost exactly a year to the day since the budget carrier which owned MAEL went bust , leaving more than 100,000 Britons stranded across Europe and in need of the biggest repatriation exercise since the Second World War.
People close to the situation said that HMRC had yet to serve a winding-up petition but confirmed that one had been discussed between the engineering group's stakeholders in the last few days as they seek to resolve its future.
MAEL said it was "not the subject of a winding-up order or any other form of administration or insolvency process".
Insiders said there was optimism that a "cleaned-up" MAEL freed of the legacy liabilities generated by the Monarch airline could have a bright future, with a large chunk of the revenues lost when the carrier collapsed already having been replaced by new customers.
Technically still a subsidiary of Monarch Airlines, MAEL is not in itself in administration but is saddled with debts totalling well over £100m, according to insiders.
Sources described that position as "unsustainable".
The bulk of that is secured debt held by Greybull Capital, the investment firm which owned Monarch for three years before administrators from KPMG were called in when regulators declined to renew its licences.
The Air Travel Trust (ATT), which helps to meet the cost of refunds and repatriation when ATOL licence-holders go bust, is thought to be owed roughly £15m by Monarch's engineering arm after the debt was transferred to it from its former parent.
Several sources said that one possibility being explored by KPMG and MAEL was to launch a Company Voluntary Arrangement (CVA), a mechanism which has been used by dozens of retailers and restaurant chains this year to reduce their obligations to creditors.
If employed by MAEL, such a move could impact on the claims of unsecured creditors including the ATT, which falls under the auspices of the Civil Aviation Authority (CAA), with Greybull also expected to write off some of its substantial debt and provide new capital to the business.
The unit's other creditors include Boeing (NYSE: BA - news) , which contributed to a rescue package for Monarch Airlines in the months before its demise, the Pension Protection Fund and PNC, an asset-based lender which is thought to be owed more than £10m.
During its tenure as Monarch's owner, the airline contributed £250m to the taxman, with more than £25m paid to the ATT in ATOL fees and bonds.
KPMG is leading the talks about MAEL's restructuring, which the company insisted in a statement on Thursday was being conducted from a position of strength.
"One year on from the failure of Monarch Airlines, MAEL has successfully stabilised operations and transitioned to a standalone business.
"It has built on its strong foundations as a leading, independent aircraft maintenance company and, under the leadership of its skilled and experienced management team, has strengthened its customer base through new contracts with customers including Thomas Cook Airlines, Virgin Atlantic and Vueling."
It added: "The company's hangars in Luton and Birmingham are now fully utilised and operating at maximum capacity, with contracted work stretching throughout 2019."
The ownership structure, it said, was "unsustainable in the medium and longer term and the review is now necessary".
"MAEL has made good progress in agreeing the terms on which the ownership structure and legacy issues will be resolved.
"As part of this process, the MAEL balance sheet will be strengthened and management are confident that the process will be finalised by the end of this month.
Chris Dare, Monarch Aircraft Engineering's chief executive, said the ownership review was "the final phase in the journey to go from being a division of Monarch Group to a successful standalone entity".
"We have enjoyed strong support from our key stakeholders which has been so important as we complete our journey," he added.
An HMRC spokesman said: "We do not comment on identifiable individuals or businesses."