The transition period ends in accordance with the withdrawal agreement. On 23 March 2018, EU and UK negotiators reached an agreement on the draft withdrawal agreement allowing the European Council (Article 50) to adopt guidelines for the framework for future eu-UK relations. On 15 January 2019, the House of Commons voted with 230 votes against the Brexit withdrawal agreement the largest vote against the British government in history.  The government may survived a vote of confidence the next day.  On March 12, 2019, the House of Commons voted 149 votes against the agreement, the fourth-biggest defeat of the government in the history of the House of Commons.  A third vote on the Brexit withdrawal agreement, widely expected on 19 March 2019, was rejected by the House of Commons spokesman on 18 March 2019, on the basis of a parliamentary convention of 2 April 1604, which prevented British governments from forcing the House of Commons to vote several times on a subject already voted on by the House of Commons.    An abbreviated version of the withdrawal agreement, in which the annex political statement had been withdrawn, consisted of the test of „substantial amendments,“ so that a third vote was held on 29 March 2019, but was rejected by 58 votes.  The political declaration provides that the EU-UK agreement on future relations will essentially be an economic and security partnership. In accordance with the political declaration, the 27 EU Member States agreed on 25 February 2020 on the negotiating mandate of the European Commission, which is negotiating on behalf of the Member States on future relations between the EU and the UNITED Kingdom.
On this basis, the EU`s chief negotiator, Michel Barnier, presented in mid-March a draft comprehensive agreement on the new partnership with the UK, to which other elements were added. You can find this and the others on the UK Task Force`s website. On 6 September 2020, the Financial Times reported that the UK government was considering drafting new laws to circumvent the protocol of the Northern Ireland Withdrawal Agreement.  The new law would give ministers the power to determine which state aid should be notified to the EU and to define which products at risk of being transferred from Northern Ireland to Ireland (the withdrawal agreement stipulates that in the absence of a reciprocal agreement, all products are considered vulnerable).  The government defended this approach and stated that the legislation was in accordance with protocol and that it had only „clarified“ the volumity in the protocol.  Ursula von der Leyen warned Johnson not to violate international law and said that the implementation of the withdrawal agreement by Britain was a „precondition for any future partnership“.  On 8 September, the Minister of Foreign Affairs for Northern Ireland, Brandon Lewis, told the British Parliament that the government`s internal market bill would „violate international law“.“  On 22 October 2019, the House of Commons agreed, by 329 votes to 299, to give a second reading to the revised withdrawal agreement (negotiated by Boris Johnson earlier this month), but when the accelerated timetable it had proposed did not receive the necessary parliamentary support, Johnson announced that the legislation would be paused.   The withdrawal agreement between the European Union and the United Kingdom sets out the conditions for the United Kingdom`s orderly exit from the EU, in accordance with Article 50 of the Treaty on european Union.
The withdrawal agreement provides for a transitional period until 31 December 2020, during which time the UK will remain in the internal market, to ensure the smooth flow of trade until a long-term relationship is concluded. If no agreement is reached by then, the UK will leave the single market without a trade deal on 1 January 2021.