The Collaborative International Dictionary of English v.0.48:

labourious \labourious\ adj.
   1. characterized by toilsome effort; same as laborious; --
      British spelling. [Chiefly Brit.]

   Syn: arduous, backbreaking, back-breaking, grueling,
        gruelling, hard, heavy, laborious, punishing, slavish,
        strenuous, toilsome.
        [WordNet 1.5]