OTTAWA - Air Canada said Monday the Canada Industrial Relations Board (CIRB) has declared ongoing strike activity by the company's 10,000 flight attendants is unlawful.

The leadership of the Canadian Union of Public Employees (CUPE) has been ordered to direct its members to return to work, said Air Canada in a news release.

According to the release, following a hearing on Sunday, the CIRB also directed individual members of the union to cease all unlawful activities and return to work.

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The union and its officers shall provide a written public notice to all members by 12 pm Eastern Time (1600 GMT) on Monday, the board declared in its ruling.

Canadian Minister of Jobs and Families Patty Hajdu issued a statement on Saturday saying that she exercised her authority under the Canada Labour Code to direct the CIRB to arbitrate the dispute between Air Canada and CUPE and end the strike, which began early Saturday.

After its flight attendants defied a government back-to-work order, Air Canada suspended plans to resume limited flying on Sunday evening. In a previous news release, Air Canada said it will resume flights as of Monday evening.

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The airline estimated that 500,000 customers' flights have been cancelled as a result.

Despite eight months of negotiations on issues of compensation increase, ground pay, improved pensions and benefits, as well as increased crew rest, Air Canada has been unable to reach a tentative agreement with CUPE.