Marx doesn't have a labour theory of value, he wrote a critique of the classical economists labour theory of value. Marx does not see value as a necessary expression of labour. He sees the distribution of labour time in society according to value relations as a peculiarity of the capitalist organisation of society.
Marx criticised the classical economists for being preoccupied with the content of value, labour time, but indifference to the _form of value_.
He meant that the form of value, ie the very fact that labour time finds its expression in exchange value, was the thing that should be investigated, because it was what was distinctive to capitalism. The blind operation of a law of value, outside of human consciousness is a strictly limited form of social relations, unique to capitalism. Assuming what they ought to explain _why it is_ that labour time attaches itself to goods in exchange as values, the classical economists missed the point about value in exchange, that it was strictly limited to capitalist societies. He was saying that their approach was ahistorical.
-- Jim heartfield