Statutory instruments are known as Secondary legislation  They make changes to the law under powers that have been specified in a previous Act of Parliament.
During the passage of a Bill, Parliament agrees to any these powers as a way of making future changes to the law.
Secondary legislation (Statutory Instruments) cannot normally amend primary legislation. It tends to ‘flesh out’ Acts with more detail and it is more easily revised, which makes it a more flexible and powerful tool for Ministers.
Statutory Instruments (normally in the form of an ‘Order’ or ‘Regulations’) are laid before the House of Commons and made under powers contained in an existing Act of Parliament.
Statutory Instruments (secondary legislation) then become law by one of two procedures – an affirmative resolution or negative resolution – depending on which was specified in the Act of Parliament under which it is made.
The affirmative resolution procedure requires the positive endorsement of both Houses of Parliament. Normally, a motion to approve a draft statutory instrument is tabled and put to the House of Commons. A statutory instrument is usually agreed without a debate or vote, but opposition Members of Parliament can force either if they are so minded. If both Houses do not indicate their backing for the instrument, it cannot not become law.
The negative resolution procedure assumes that a statutory instrument is acceptable unless action is taken to the contrary. Normally, a statutory instrument under this procedure is made by a Minister and becomes law immediately. A motion to annul the instrument may be tabled in either the House of Commons or House of Lords for a period of 40 days after the day on which the order is first made.
If an annulment is then agreed, the Statutory Instrument then ceases to have been law.
The Joint Committee on Statutory Instruments may consider a Statutory Instrument, including taking evidence, and report to either House to aid the scrutiny of secondary legislation.