The House of Lords is mainly nowadays a revising chamber with the ability to ask the Commons, and the Government, to think again by passing amendments. But on secondary or delegated legislation - implementing measures in an act or primary legislation - the Lords has a veto. Peers cannot amend such a measure, only approve or reject it. The Parliament Acts of 1911 and 1949, establishing the ultimate supremacy of the Commons, do not apply to secondary legislation. Hence the description of the possible Lords motion against the Government's tax credit cuts as 'fatal': if it were to pass, the cuts would be halted and the Government would have to start all over again.
That power to kill off secondary legislation is very rarely used precisely because it is so final - unlike the exchanges between the two Houses on bills or primary legislation. (These are known as 'ping-pong' and allow time for compromise and further thoughts.) The Lords has usually acted with restraint because it is the unelected chamber - though it has been more assertive since the departure of most of the hereditary peers in 1999. More often on secondary legislation, a non-fatal motion is tabled expressing concerns about a measure. Even where fatal motions can be tabled to secondary legislation they are not always debated, let alone voted upon. The Prime Minister has also argued that the change to tax credits is a financial measure and it is therefore the sole prerogative of the Commons to decide upon.
The current row has been intensified, not only because of the controversial nature of the cuts in tax credits, but because of the party imbalance in the Lords. Labour and Liberal Democrat peers can together easily outvote the Conservatives, and they have been doing so regularly since the general election. These defeats not only infuriate senior ministers but also pose an acute dilemma for the Opposition party managers in the Lords about how often to use this power to defeat the Government. Hence the talk amongst Tory MPs of creating even more peers on top of the large number of new peers announced in August.
While the threatened fatal motion is highly unusual, it is yet another example of the current unstable and volatile position in the House of Lords. The Prime Minister has previously ruled out any further attempts at reforming the Lords, but threats to use the Lords to kill off tax credit cuts might be just a taster of the troubles ahead for the Government and Parliament under a small majority.
Peter Riddell is the director of the Institute for Government
This blog first appeared on the Institute for Government blog, and can be read here