According to U.K. news reports, the Public and Commercial and Services union is struggling to restore its online service as its annual conference gets under way.
The union's website has been hit by traffic 1,000 times its normal level since May 11, officials say. The site is still unavailable, other than a static home page.
The PCS said it had been the victim of a "sustained and well-organized" attack that had seriously affected access to the site.
The union, which is made up primarily of government workers, is holding its annual conference in Brighton and is set to announce a ballot of members that could lead to a strike over public spending cuts.
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