Brexit A to Z: Key terms explained

A cross-party group of MPs - supported by 21 Conservative rebels - supported allowing the discussion of a new law which would require a delay to Brexit unless there was a deal.

In response, Boris Johnson announced his intention to trigger a snap general election and his party withdrew the whip from rebels including two former chancellors.

Here, the PA news agency has an A to Z guide of Brexit terms that have dominated the agenda for the past two years - and will be heavily used in days to come.

Article 50: The process that started the ball rolling. Following the Leave vote at the referendum, the UK invoked Article 50 of the Treaty on European Union on March 29, 2017 when Theresa May wrote a letter to European Council president Donald Tusk. Subsection three says that a country can leave two years after triggering Article 50 unless an extension is agreed.

Backstop: The backstop is effectively an insurance arrangement required by the EU to prevent customs or border checks between Northern Ireland and the Republic of Ireland. A bone of contention, the backstop says that until a broader trade agreement is struck between the UK and EU, Britain must remain within the European customs area.

Customs Union: The topic at the heart of discussion about a future relationship between the UK and EU, a customs union is a group of countries which abolish import tariffs and quotas on goods traded among them. The rules of the EU customs union prevent members from striking free trade deals with other countries, which advocates of Brexit have said is among the main advantages.

Dexeu: The Department for Exiting the European Union was formed by then Prime Minister Theresa May less than a month after the Brexit vote. There have been three Brexit secretaries in the last two years; David Davis, who resigned after opting not to back Mrs May's plan, Dominic Raab, who left for similar reasons, and Stephen Barclay, who holds the post currently.

European Research Group: A group of Conservatives who are defined by their strong Eurosceptism and formerly led by Jacob Rees-Mogg, the ERG previously favoured a looser Canada-style free trade agreement but many members have said they would be content to see a no-deal Brexit if that proves impossible.

Fixed-term Parliaments Act: A law introduced in 2011 which changes the way general elections are called, the Fixed-term Parliaments Act means the power to call a nationwide poll is no longer in the hands of the Prime Minister. To call an election, two-thirds of the House of Commons need to back the idea.

Great Repeal Bill: The idea of a Great Repeal Bill was first mooted by Mrs May in 2017, which would see EU law transferred onto the UK statute book to ensure what Mr Davis called "a calm and orderly exit". In the end, the law was renamed the European Union (Withdrawal) Act and has been amended to reflect the delayed exit day.

House of Commons: The site of most of the Brexit drama of late and that to come, the House of Commons will be where a potential delay to Brexit will be discussed, a potential general election voted on and all before it is closed down next week.

Independence Day: First it was March 29, then it became either April 12 or May 22 and now Britain is due to leave the European Union on October 31.

Johnson: The current Prime Minister, who was a key player in the Brexit campaign, is now the man charged with leading the United Kingdom out of the EU. He entered 10 Downing Street saying "we are going to fulfil the repeated promises of Parliament to the people and come out of the EU on October 31, no ifs or buts". He added: "The doubters, the doomsters, the gloomsters - they are going to get it wrong again."

Keir Starmer: Labour's shadow Brexit secretary who has repeatedly attempted to hold the Conservatives' feet to the fire over Brexit - including asking then-Brexit secretary David Davis 170 questions about the Government's Brexit strategy in 2016.

Liability: Britain has been warned that it will be forced to pay a £39 billion "divorce bill" to honour commitments made during its EU membership but Mr Johnson has said the money would not be due in the eventuality of a no-deal Brexit. Guy Verhofstadt, the European Parliament's Brexit co-ordinator, has warned: "If the UK doesn't pay what is due, the EU will not negotiate a trade deal."

May's deal: Also known as the Chequers Agreement, the so-called May deal was a white paper for the future relationship between the EU and UK covering economic partnership, security, cooperation and institutional arrangements, and committed to no hard border between the UK and Ireland. It was rejected by the EU in September 2018 and a new withdrawal agreement and a political declaration was rejected in the Commons.

No deal: Should Britain and the EU fail to reach an agreement before the egg-timer runs out, Britain will leave on a no-deal basis. Mrs May said that "no deal is better than a bad deal", but businesses have expressed concern about the impact of a cliff-edge Brexit.

Opt-outs: European Union law is in place across all member states, but certain opt-outs have been granted over time. The UK and Ireland were granted one to not enforce the Schengen Agreement which abolished border controls within member states, while the UK was also granted an opt-out from the Economic and Monetary union which led to the euro.

Prorogation: The term for the formal end of the parliamentary session, which was widely used last week when Mr Johnson said he would bring an end to the parliamentary term in September. While Parliament is prorogued, MPs and peers cannot formally debate policy and legislation or make any laws of their own.

Queen's Speech: The start of the parliamentary term where the sovereign reads the plans for the government in the forthcoming year. The Queen's Speech was cancelled in 2018 with Andrea Leadsom saying it would give "MPs and peers the maximum time possible to scrutinise legislation taking the UK out of the European Union".

Revocation: The Article 50 letter can be withdrawn by the UK unilaterally, without the need for EU agreement, leaving Britain free to continue as a member on its current terms. A petition set up to halt the Brexit process garnered six million signatures.

Standing Order Number 24: SO24 was used by the cross-party group to seek an emergency debate which, following a successful vote, allowed them to control the Commons business on Wednesday, guaranteeing time to debate a new law to block a no-deal Brexit.

Transition: The negotiated Withdrawal Agreement included a transition period of two years where the UK would move from being an EU member state to being outside. In one draft of the agreement, the transition period would run until December 2020 and could be extended by up to two more years. During this period, the UK would have to abide by EU laws but have no seat at the table. It would not apply in the event of a no-deal Brexit.

Union: The continuing debate about Brexit has raised questions about the future of the Union after Scotland and Northern Ireland voted for remain while Wales and England voted to leave. Scottish first minister Nicola Sturgeon said Scots should have the "right to determine our own future".

Veto: Any extension to Brexit has to be agreed unanimously by the heads of state of the remaining 27 countries, with all having a veto. Equally, should an independent Scotland attempt to join the European Union at a future date, then this decision would have to be agreed unanimously.

WTO Rules: Should the UK leave without a deal, the legal basis for the free movement of goods between Britain and the EU would disappear and instead the UK would have to abide by World Trade Organisation (WTO) rules. This would mean a new tariff regime as well as new customs and regulatory checks.

X -Cross party agreement: There have been a number of examples of parties working together to further their cause for either Leave or Remain: the Green Party and Plaid stood aside to give the Liberal Democrats a better shot at the Brecon and Radnorshire by-election, while there have also been calls for the Brexit Party to join forces with the Tories in the event of a general election.

Yellowhammer: A codename used by the Government relating to no-deal planning, Operation Yellowhammer detailed the "most likely aftershocks" of the UK crashing out of the EU. Published by The Sunday Times, the documents warned that Britain would be hit with a three-month "meltdown" at its ports, a hard Irish border and shortages of food and medicine if it leaves without a deal.

Zero: It could be argued there is next to no certainty about what will happen over the coming days, with question marks over the future of Brexit negotiations, the Johnson government and the position of those who rebelled against the party.