Canada Post employees announced last October they would take the Crown corporation to court over the cost-cutting plan to end delivery and install community mailboxes across the country.
The union's court filing is signed by six other organizations representing retired and handicapped citizens.
The groups say eliminating home delivery discriminates against the elderly as well as people with mental and physical handicaps because it limits their access to a public service.
Union president Denis Lemelin says he is willing to take the case to the Supreme Court of Canada if need be.
The union represents 54,000 workers and says the cancellation of home delivery will eliminate up to 8,000 jobs.
Canada Post has yet to reply to the union's court filing.
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