These two are not spelling variants but two different words: practice is the correct spelling of the noun and practise should only be used for the verb. So, in British English at least, you always spell practised, practising and so on with an <s>. But in many cases the two are confused, so that people frequently use them the wrong way round. The charity Mencap has launched an online Spelling Bee for primary and secondary schools to give children the opportunity to ‘practice’ their spelling. Sarah Brown, wife of then Prime Minister Gordon Brown, came under fire from the Twitter language police when, having returned from a trip to a kids’ bowling party, she confessed that she would have ‘to practice more’.
The source of both words is the verb practise, which was derived from a French word practiser ‘to strive’ in the 15th century, pronounced with the stress on the second syllable. The noun was derived from the verb and was originally spelled practise; it was subsequently spelled with the –ice ending to bring it into line with words like justice, cowardice and malice, where the –ice ending derives from the Latin ending –itia. While, in practice (not practise), this allowed a written distinction between the two words, the two spellings were interchangeable until the 18th century. To make matters worse, the pronunciation of the verb also changed, with the stress shifting to the first syllable, so that the two words became interchangeable in speech too. Since the 19th century, the single spelling practice has been used for both noun and verb in the US, making things a whole lot simpler. What would we lose if British spelling were to follow suit?