The powers of the House of Lords are limited by a combination of law and convention.
The Parliament Acts, although rarely used, provide a way of solving disagreement between the Commons and the Lords.
Parliament Acts: background
Until the early years of the 20th century, the House of Lords had the power to veto (stop) legislation.
However, this arrangement was put under pressure when the House of Lords refused to pass David Lloyd-George's 'people's budget' of 1909. Eventually, the budget was passed after a general election in 1910; a second general election was then fought on the issue of reform of the House of Lords.
Parliament Act 1911
The result was the Parliament Act 1911, which removed from the House of Lords the power to veto a Bill, except one to extend the lifetime of a Parliament. Instead, the Lords could delay a Bill by up to two years. The Act also reduced the maximum lifespan of a Parliament from seven years to five years.
Parliament Act 1949
The Parliament Act 1949 further reduced the Lords' delaying powers to one year.
The Parliament Acts define the powers of the Lords in relation to Public Bills as follows.
Money Bills (Bills designed to raise money through taxes or spend public money) start in the Commons and must receive Royal Assent no later than a month after being introduced in the Lords, even if the Lords has not passed them. The Lords cannot amend Money Bills.
Other Commons Bills
Most other Commons Bills can be held up by the Lords if they disagree with them for about a year but ultimately the elected House of Commons can reintroduce them in the following session and pass them without the consent of the Lords.
Bills not subject to the Parliament Acts
Bills prolonging the length of a Parliament beyond five years
Bills sent up to the Lords less than a month before the end of a session
Bills which start in the Lords
The Salisbury Convention
The Salisbury Convention ensures that Government Bills can get through the Lords when the Government of the day has no majority in the Lords.
In practice, it means that the Lords do not vote down a Government Bill mentioned in an election manifesto.