A union representing tens of thousands of public servants says the federal government now owes more than $10 million in union dues due to ongoing problems with the Phoenix payroll system.

In a letter obtained by CBC News through access to information legislation, the Public Service Alliance of Canada expresses its concern for the "substantial reduction in dues" the union has received.

The letter — dated Oct. 4, 2017, and addressed to the Treasury Board Secretariat — states the government owes the union approximately $7.5 million.

PSAC officials now confirm the amount is more than $10 million and say no money has been remitted to the union, even though the letter requested "an immediate remittance" to offset the substantial debt.

'Affects our ability to carry on operations'

"There has been no change in our membership demographics or compensation which can explain or account for this discrepancy. A financial shortfall of this magnitude affects our ability to carry on operations and properly fulfill our statutory mandate," the union's letter states.

In the 2018 federal budget released last month, there are provisions to provide unions with money.

"The government will also take action to reimburse missing and inaccurate dues that are owing to public sector unions," the budget reads.

The budget has also committed hundreds of millions of dollars to address Phoenix issues, hire additional staff and eventually work toward a new pay system.

PSAC officials would not say how much the government has agreed to remit, but to date, no money has been recovered.

"PSAC's top priority still remains doing everything we can to help get our members paid correctly," said Robyn Benson, the union's president. "Our union filed an interference complaint with the Federal Public Sector Labour Relations and Employment Board because the government owes PSAC over $10 million."

No exact figures

Ongoing Phoenix accounting issues mean that neither the government nor PSAC know exactly how much is owed to PSAC, according to the union.

"The $7.5 million was our best estimate in October, but that estimate has grown to over $10 million as we've done more research," said Michael Aubry, a PSAC communications officer. "The issue, of course, is that Phoenix is causing errors for the union, the government and federal workers, so there's no easy way to do a proper accounting of dues owed."

Aubry also said the system often cannot properly track when workers join or leave the federal public service. That means the union may not have an exact cost estimate of the dues it's owed until Phoenix is fully functional.

This isn't the first time a union has called on the government to pay dues lost to the problem-plagued Phoenix pay system. Last year, the Professional Institute of the Public Service of Canada was also seeking unpaid dues.

PIPSC officials would not provide an update on how much it's owed.