Sharia law is "the path that must be followed by a Muslim".
It brings together elements from the Koran and the Hadith (a collection of the deeds and words of Mohammed), plus judges' rulings from Islam's first centuries. It was fixed by about the 10th century, and contains detailed instructions for practically every aspect of life.
In the West, it is most famous for its penal code: the prescribed punishments for sexual offences, which include stoning; for theft, which include amputation; and for apostasy, for which the punishment is death.
Much more important for most Muslims, however, are the parts of sharia that relate to the status of women, to contracts and to family law.
These include provisions that allow men several wives and that enshrine, in law, the inferiority of women.
Women can be divorced merely by their husbands reciting "I divorce you" three times; their testimony is worth less than that of men; and they cannot marry a non-Muslim man - although it is permissible for a Muslim man to marry a non-Muslim woman.
It is parts of sharia such as these that come into immediate conflict with Britain's secular law, which is committed to treating all citizens equally. But it is those provisions which Muslim clerics most want to cordon off from any secular influence.