The winding‑up petition brought by HMRC appears to relate to an alleged dispute over unpaid VAT.
Robin Hughes, branch secretary of Unison South West Ambulance, said he was “disturbed” by the situation.
“It begs the question of whether this area is being properly regulated, and whether the people who commission these services are paying close enough attention to what is going on,” he said.
Pillinger has been approached for a comment in relation to the claim by HMRC that his company owes more than £1m in VAT.
He told staff in a memo on Wednesday that the company faced some financial pressures as a result of a reduction in how frontline A&E staff operate.
Pillinger described the HMRC winding-up petition as a “serious matter … being actively managed with support of partners”.
He added: “I am pleased to confirm that all critical financial commitments, including wages … through the end of May, are fully supported.
“Importantly, there has already been strong interest from a number of established operations within our sector regarding a potential investment in or acquisition of the business.”
Pillinger also promised minimal disruption to staff and patients.
In a statement HMRC said it could not comment on businesses due to taxpayer confidentiality laws but added: “We take a supportive approach to dealing with customers who have tax debts and only file winding-up petitions once we’ve exhausted all other options, in order to protect taxpayers’ money.”