Diplomats stationed at 12 Canadian missions walked off the job Thursday in a significant escalation of work stoppages that could affect the Prime Minister's trip to Europe next week.

Several of Canada's largest foreign missions are among those targeted by the job action, including London, Tokyo, Paris and Hong Kong. Immigration officers in some of Canada's largest processing centres in China and India also walked out.

It is the largest withdrawal of services by members of the Professional Association of Foreign Service Officers since the union declared it was in a legal strike position this spring. PAFSO says it is now targeting Canada's most important international files and all major travel by Canadian politicians, including Prime Minister Stephen Harper's upcoming trip to Europe for the G8 summit.

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Several hundred employees at the Department of Foreign Affairs and International Trade headquarters in Ottawa also walked off the job temporarily at 2 p.m. in support.

PAFSO says its members are paid substantially less than other public servants who do similar work in Canada. The union has been gradually escalating job actions in an effort to force the federal government to offer bigger pay increases.

Union president Tim Edwards said on Thursday that the two sides talked twice this week but the Treasury Board did not change the pay offer, which has been on the table since negotiations began.

"In light of that fact, we rejected it on the spot and walked away from the table," Mr. Edwards said.

Many of the foreign service officers who would normally be involved in preparations for next week's G8 summit are off work, he said.

"It essentially means the Prime Minister and his staff will not have the support or the preparation that they are accustomed to for these meetings," Mr. Edwards said. "They will be under-prepared for these meetings."

A spokesman for the Prime Minister's Office said Mr. Harper "is always fully prepared for his meetings and looks forward to a productive trip."

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The job actions will likely cause a significant backlog in processing visas and permanent resident applications and limit progress on Canada's trade and investment priorities, Mr. Edwards said.

NDP foreign affairs critic Paul Dewar said he is concerned about a "cascading negative effect" on Canada's international engagements.

"Everything from trade to peace and security issues is really dependent upon our foreign service officers," he said. "It's not about just bending to them, it's about negotiating in good faith."

Asked about the impact of the job action on Canadians, a Foreign Affairs spokeswoman said services have not yet been affected and the government will work to limit any possible disruptions.

The union says its 1,350 members are paid $3,000 to $14,000 less than their counterparts in Canada. It is asking for changes to the Treasury Board's pay scales for diplomats to bring them up to the same levels.

For example, an experienced foreign service officer on the level-2 pay scale makes about $83,000 a year, which would increase to about $87,000 under the Treasury Board offer, Mr. Edwards said. That would still be about $11,000 less than the salary of employees at the same level in the commerce division.

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A spokesman for Treasury Board President Tony Clement said the government made a fair offer.

"The foreign service is a highly sought after and well-paid posting," Matthew Conway wrote in an e-mail. "It is unfortunate that the union is attempting to take their labour demands out on Canadians. We expect that any job action will respect the bounds of the law."

Mr. Conway added that foreign service officers receive additional benefits, including partial reimbursement for dry cleaning, help in shipping household belonging and sending a vehicle abroad and an incentive allowance for working outside the country.

Foreign service officers abroad can receive a "foreign service premium" of $6,600 to $31,300, depending on their seniority and the number of dependants accompanying them on a posting. A smaller, post-specific allowance is meant to help diplomats with the cost of travel.

Mr. Edwards said the union made several concessions in bargaining, including accepting a cap on annual pay increases and the loss of severance for resignations and retirements. The union has been without a contract since June, 2011.

Editor's note: An earlier version of this article incorrectly stated that foreign service officers receive a vehicle when they go abroad.