The law has been rushed through both chambers of parliament over the past week, and was opposed by the government, which has said it could limit its room for negotiation with the European Union.
As a result of the law, the government now must put forward a motion on Tuesday in parliament, setting out the delay, that it will request at an EU summit on Wednesday.
MPs will be allowed to vote on this, and make their own suggestions with regard to the length of the delay.
It allows the government to seek any extension from 22nd May onwards. Prime Minister Theresa May has asked for a delay until June 30, but ultimately this is up to EU leaders and it could be longer.