The country's largest federal public service union says if a deal isn't reached with the federal government by 9 p.m. EDT on Tuesday, it will launch a strike this Wednesday.

The Public Service Alliance of Canada says some 155,000 employees are prepared to walk off the job, including 35,000 workers from the Canada Revenue Agency.

Mediated contract negotiations between the union and the Treasury Board continued over the weekend in what the union described as the government's final chance to reach a deal.

Chris Aylward, the union's national president, said at a news conference Monday morning that some progress has been made — but not enough to call off a strike.

He said CRA workers are also back at the bargaining table Monday and Tuesday after announcing their own separate strike mandate on Apr. 7.

Unless they reach their own deal by Tuesday evening, those workers will strike, too, he said.

"Despite some progress at the bargaining table, our members are frustrated that while negotiations drag on, they continue to fall behind," said Aylward.

"We've already been at the table for nearly two years and these workers can't wait any longer. That's why we're setting a clock on this round of bargaining."

The biggest sticking point in the talks appears to be pay, with the union calling for raises to keep up with the rising cost of living and historic inflation.

The government offered a roughly two per cent average wage increase each year over a five-year period, while the union has pushed for annual raises of 4.5 per cent for the next three years.

The union also wants to put on the table greater limits on contract work, more anti-racism training and provisions for remote work.

Should the union strike on Wednesday, many federal services, from tax processing to passport renewal, could be affected — with departments and agencies signalling which essential services will continue during a strike and which may be disrupted.

"We want to have an impact on the government. We will try to have as least impact on Canadians as possible," said Aylward.

Aylward said if they go on strike, federal public servants will be picketing at strategic locations across the country.