In Westminster Parliaments, a reasoned amendment is an amendment to the wording of the motion on the reading of a bill, which turns the motion from one progressing the bill into one rejecting it. It is sometimes called a wrecking amendment, but the term wrecking amendment more often denotes a type of amendment to the bill itself, rather than to a motion on reading the bill. The term "reasoned amendment" reflects the fact that the amended motion specifies reasons for rejecting the bill.

Consider the second reading in the House of Commons of the United Kingdom of a bill which has 200 supporters and 300 opponents.

A typical process with no reasoned amendment might be:

The corresponding process with a reasoned amendment might be:

With no reasoned amendment, the precise reasons for the bill's rejection may be inferred by reviewing the transcript in Hansard, but are not formally recorded in the Journal of the House of Commons. With a reasoned amendment, the motion recorded in the Journal will give the reasons.

For instance, in 1998 the House of Lords passed the following reasoned amendment:

Leave out all the words after "That" and insert "this House declines to give the European Parliamentary Elections Bill a Second Reading on the grounds that it includes an undemocratic 'closed list' system providing for the selection of MEPs by party choice, an approach which would end the historic right of the British people to choose the candidates they wish to be elected, a step for which the House notes with great concern no mandate was sought or given at the last general election".

In this case, the Commons invoked the Parliament Acts 1911 and 1949 to override the Lords' rejection of the bill, which later became law as the European Parliamentary Elections Act 1999.