This is a point on which British and US usage differs. Here is advice from two usage guides.
R.W. Burchfield, The New Fowler's Modern English Usage (OUP 1998) :
Backward(S) in most adverbial uses : backward and backwards are interchangeable, but usage varies subtly from person to person and from region to region. It is broadly true to say that in North America backward seems to be somewhat more usual than backwards and in Britain the other way round. As an adjective, the only form used is backward (without a backward glance).
Bryan A. Garner, A Dictionary of Modern American Usage (OUP USA 1998) :
Directional words A : The suffix -ward(s) In American Usage the preferred practice is to use the -ward form of directional words as in toward, forward, westward. Words ending in -ward may be either adjectives or adverbs whereas words ending in -wards, common in British Usage, may be adverbs only. Two exceptions in American Usage are the adverbs afterwards and backwards which are almost universally used in preference to afterward and backward. It's anomalous that most people say forward but backwards.
The Oxford English Dictionary records that backwards has been in use in English since the 16th century.