Following the UK's accession application, seven countries have applied to join CPTPP: China, Costa Rica, Ecuador, Indonesia, Taiwan, Ukraine, and Uruguay.

[12][13] In January 2018, the government of the United Kingdom stated it was exploring membership of the CPTPP to stimulate exports after Brexit and has held informal discussions with several of the members.

[28] Japan had expressed support for the UK's potential entry into CPTPP in 2018,[29] and as 4th CPTPP Commission (2021) chair, Japan's minister in charge of negotiations on the trade pact, Yasutoshi Nishimura, expressed hope on Twitter that Britain will "demonstrate its strong determination to fully comply with high-standard obligations" of the free trade accord, and mentioned that "I believe that the UK's accession request will have a great potential to expand the high-standard rules beyond the Asia-Pacific.

[33][34] This meant that the UK had continuity free trade agreements with 6 CPTPP members, these were: Canada, Chile, Mexico, Peru, Singapore, and Vietnam.

[45][6] After the UK became a signatory on 16 July 2023, the accession protocol allowed 15 months for each member nation to ratify the agreement, which would then take effect 60 days later.

[46][47][9][48] In an article published on 4 November 2023 in the UK newspaper the Daily Express, the UK Business and Trade Secretary Kemi Badenoch confirmed that the process of ratification for CPTPP accession was to commence shortly – stating that "in the coming weeks, we will bring forward a Bill to confirm our membership of CPTPP and ensure businesses can start seizing the historic opportunities the deal offers.

"[65] On 7 November, the UK government (via a proclamation from King Charles III) announced in the State Opening of Parliament that a bill would be brought forward to ratify the UK's accession to the CPTPP – saying that "My ministers will take steps to make the economy more competitive, taking advantage of freedoms afforded by the United Kingdom's departure from the European Union, a bill will be brought forward to promote trade and investment with economies in the fastest growing region in the world.

"[66] The name of the bill was confirmed in press briefings ahead of the King's Speech to be the "Trade (Comprehensive and Progressive Agreement for Trans-Pacific Partnership) Bill"[67][68] The fact that an Act of Parliament is required in order to bring effect to the CPTPP accession, means that there are clauses agreed to within the accession protocol that require primary legislation in order to bring the agreement fully into place.

[48][70] Timeline 9 of 11: 24 December 2024 The United Kingdom attended its first CPTPP meeting after signing the accession protocol on 15 November 2023 in California, on the sidelines of the 30th Asia-Pacific Economic Cooperation summit in San Francisco.

[79] In a report released with the 2023 Autumn statement, the Office for Budget Responsibility calculated that Britain's economy would gain just 0.04% to GDP in the "long run", which it defined as after 15 years of membership.

[81] Business and Trade Committee chair Liam Byrne criticised the Government for "dodging" scrutiny of the UK's accession to CPTPP, demanding MPs should have had more time to debate the treaty.

[86] The National Institute of Economic and Social Research estimated that the negative impact of Brexit gradually escalates, reaching some 5-6 per cent of GDP or about £2,300 per capita by 2035.

[87] Trade Journalist Alan Beattie states that the 5 per cent of GDP long-run cost of leaving the EU single market and customs union is not compensated by trade agreements such as CPTPP, regardless of who joins it in the future, pointing out that the long run collective gain of CPTPP, the Australia, and New Zealand deals are worth just 0.2 per cent to UK GDP.

Beattie argues that CPTPP, and any future trade agreement, that create obstacles to realigning with the European single market are harmful to the economy of the United Kingdom in the long term.

[91][92] Further concerns were raised by trade unions over clauses in the deal that will allow large companies to sue the UK government behind closed doors if they believe their profits have suffered from changes to laws or regulations.

[93] In March 2023, British Secretary of State for Business and Trade Kemi Badenoch argued Britain's decision to join the CPTPP gives it a strengthened presence and influence in the Asia-Pacific region which is rapidly growing in significance both economically and politically.

[94] Additionally, Badenoch also claimed the deal is the most significant commercial agreement signed by the UK since Brexit, with a potential to grow in importance as the rise of Pacific Rim countries continues, with applications from China and Taiwan highlighting further market opportunities that Britain could gain access to through CPTPP.