Britain's membership to the Comprehensive and Progressive Agreement for Trans-Pacific Partnership (CPTPP) has been seen as a major diplomatic win for Japan.
The signing of the deal in Auckland, New Zealand on Sunday by UK trade chief Kemi Badenoch will mark Britain's entry into the Pacific bloc, 11 years after it was first agreed upon by 11 nations.
Japan had led the UK's accession process and worked tirelessly to convince London to join the bloc, as part of its efforts to guard against the growing influence of China in the Indo-Pacific region.
The Japanese government saw British membership as a way to balance China's economic and political influence over other Asian members of the CPTPP, such as Vietnam, Singapore, and Malaysia.
The CPTPP was initially established in 2018, but the United States had been lined up as a 12th founding member until then-presidential candidate Donald Trump
denounced the deal as the "rape of our country" in June 2016.
The US eventually pulled out of the talks after Trump was sworn into office.
For Japan, the CPTPP is a crucial tool in recalibrating Western thinking toward its own strategic aims in the Indo-Pacific, particularly in bolstering economic and military defenses against China.
Britain's membership to the bloc is seen as a way to counterbalance China's influence and protect the interests of the West in the region.
Britain's entry into the CPTPP is also notable because it comes at a time when the country's trade relationship with the European Union is faltering.
The signing of the CPTPP deal followed in a matter of weeks after Britain threatened to break its recently-signed trade agreement with the EU, spooking CPTPP partners and raising questions about UK reliability.
The signing of the CPTPP deal is also significant because of the tensions it has caused within the bloc.
China and Taiwan have both made applications to join the pact in late 2021, and their rival candidacies must be addressed by existing members of the bloc.
It is expected that the UK will veto China's application outright.
For the UK, the In early 2021, UK Trade Secretary Liz Truss filed Britain's application to join the Comprehensive and Progressive Agreement for Trans-Pacific Partnership (CPTPP) with the aim of completing the deal within six months to a year.
However, the political chaos that gripped the UK throughout 2022, including the fall of two prime ministers, meant that the deal took much longer than anticipated.
In June 2022, Truss brought forward the Northern Ireland Protocol Bill, which put the powers to tear up the protocol negotiated as part of the original Brexit deal into domestic law.
This move was met with concern from Japan, which co-ordinated a group of CPTPP members to tell the UK to drop the protocol bill and stick to international agreements.
A delegation of Japanese diplomats and trade experts traveled to London in October 2022, just after Truss' resignation from Downing Street, to deliver a clear message to the incoming PM Rishi Sunak's government: there must be no shying away from an international treaty.
Canada, another leading CPTPP member, also supported the UK's accession to the bloc, with a senior Canadian official stating that it was "incredibly important" for the UK to respect international agreements it signed.
As Japan took up its presidency of the G7 in early 2023, Prime Minister Fumio Kishida visited London in person to sign a new defense pact and to "jointly tackle the remaining issues regarding the [UK] accession" to CPTPP.
Kishida also raised the UK's CPTPP accession drive and the regional issues around China with Canadian leader Justin Trudeau during his visit to Canada.
The UK's Trade Secretary, Andrea Leadsom, met with Japanese Trade Minister Nishimura Yasutoshi at the World Economic Forum in Davos, Switzerland, and expressed her gratitude for Japan's support in getting Britain's accession to CPTPP over the line.
Liz Truss, who preceded Leadsom as Trade Secretary, frequently expressed her love of Japan and appreciation for the country's assistance.
For Tokyo, the benefits of UK membership of CPTPP are clear.
As Japan Britain's membership in the Comprehensive and Progressive Agreement for Trans-Pacific Partnership (CPTPP) has strengthened Japan's efforts to counter Chinese influence in the region.
According to Simon Fraser, a former British diplomat and managing partner at consultancy Flint Global, Britain's membership "is a not insignificant expression of the Indo-Pacific tilt in British foreign policy" set out in the Integrated Review, a defense and security strategy launched by the UK in 2021.
Britain's presence in the CPTPP has made it easier for Japan to assert its position in the region.
A Japanese diplomat was quoted as saying that they "hope that makes China more prudent against doing something bad." However, Peter Ricketts, another former UK Foreign Office chief and Britain's first national security adviser between 2010-2012, has warned that Britain's membership does not signal a significant shift in diplomatic, economic, or security efforts towards Japan and the Indo-Pacific.
The UK's support for Ukraine in pushing back against Russia's invasion and its "limited" resources mean that the UK's focus may not be entirely on the Indo-Pacific region.
With the Labour Party rising in the opinion polls ahead of an expected general election in 2024, it is also unclear whether the next UK government will prioritize Britain's role in the Indo-Pacific region.
Despite this, the role played by Britain as a new member of the CPTPP is likely to be key as members begin to consider the next set of applications to their growing club, including one from Beijing.