Parks Canada employees across the country have received letters warning they're not allowed to criticize the agency or the federal government.

The directive comes as the agency cuts hundreds of jobs or curtails work hours.

"I am aware that during this time of significant transition, the concept of loyalty can have a very particular meaning. However, as employees of the public sector, our duty is to support the elected government," employees were told.

Workers are not supposed to speak about the cuts, whether at meetings, forums or through social media. Only designated people are allowed to deal with journalists.

Anyone who has an issue is supposed to go through internal processes, like filing a grievance, or talk with a supervisor, human resources manager or the Parks Canada ombudsman.

A copy of the letter, obtained by CBC News, is signed by Chip Bird, field unit superintendent in Cape Breton.

But thousands of federal workers got a similar warning.

Eddie Kennedy, national executive vice-president of the Public Service Alliance of Canada, said the letter went out to 4,800 agency employees around Canada three weeks ago.

"If you're in a coffee shop and you're criticizing the Harper government and there's someone sitting beside you and they know you work for a government department, technically you're in violation of the code of ethics," he told CBC News on Thursday.

The letter says the "duty of loyalty" to Parks Canada is spelled out in that code of ethics.

"The duty of loyalty includes the duty to refrain from public criticism of the Government of Canada when speaking as an employee of the agency. Breaching the duty of loyalty may lead to disciplinary action," it states.

Kennedy said he's not aware of any employee being disciplined for speaking out.

'Gag order'

One worker agreed to speak to CBC News as long as he wasn't identified.

"I only have weeks to retirement," the employee said. "It's the Harper government. What can you say? It's a gag order. It has to be challenged."

If there is a complaint against an employee, the union plans to fight it. Kennedy said they will turn to the Canadian Human Rights Commission if necessary.

Kennedy said other government departments are including similar language in their codes of ethics.

It means the government's line is the only one getting out, he said.

"If the government's doing cuts, the only message you're getting on that are their prepared media lines on the cuts. And most government departments aren't going to come out and say, 'Our cuts are going to reduce the services to Canadians, or they're going to close this office, or you're going to have a harder time qualifying for unemployment insurance.'"

The letter says there are "exceptional circumstances" to this rule, though it doesn't give specifics: "To determine whether public criticism is acceptable, the duty of loyalty must be balanced against other interests, such as freedom of expression."

Union representatives who are designated spokesmen, like Kennedy, "enjoy a much greater scope of freedom of expression."