Since being signed in 2018, the Comprehensive and Progressive Agreement for Trans-Pacific Partnership (CPTPP) has rapidly become one of the world's most important free trade deals. Its 11 signatories account for some 500 million consumers and 13.5 per cent of global GDP.

This makes it the world's third-largest free trade area by GDP, behind the EU and the US-Mexico-Canada agreement, despite having been put together relatively quickly following the withdrawal of the US from its predecessor, the Trans-Pacific Partnership (TPP). But it has the potential to grow even bigger in the coming years.

Several nations have expressed interest in joining the CPTPP in recent months, with varying degrees of commitment. So which countries could be closest to adding their signatures, and what impact would such moves have on world trade?

China

Perhaps the biggest prize for the CPTPP would be the accession of China into the deal. This would be a marked change in approach for many of the existing nations, as the previous TPP deal, with the US at the helm under the administration of Barack Obama, had specifically sought to exclude Beijing from the deal and act as a counter to increasing Chinese influence in the region.

But with the US no longer involved, the addition of the world's second-largest economy would provide a major boost to the deal, instantly growing its share of global GDP to around 28 per cent. According to the Peterson Institute for International Economics, the addition of China would increase the expected economic gains fourfold, from $147 billion annually to $632 billion.

China itself has expressed interest in joining recently. While the country has been skeptical of multilateral deals in the past, its role in the Regional Comprehensive Economic Partnership (RCEP) indicates its attitude is shifting. 

State-owned news outlet China Daily has said, for instance, that joining the deal would continue the nation's policy of opening up to foreign markets and competitors. It added: "One desirable consequence of this is that Chinese domestic firms, including those in services and high-tech industries, will be forced to upgrade their products to compete with world-class firms."

However, there are significant barriers to such a move. Signing up to the CPTPP would require China to make major changes to areas such as intellectual property, labor standards and state aid rules. The nation's human rights record and ongoing trade war with Australia may also be issues for existing members - though these have not prevented it joining RCEP.

United Kingdom

On the face of things, the UK may seem like an unlikely member of the CPTPP, being hugely geographically distant from the region. However, in its new post-Brexit environment, the country's government is keen to seek out new opportunities for global trade, and it has strong ties to several CPTPP nations, including Commonwealth members Australia, New Zealand and Canada.

Indeed, the UK's plans to join are currently at the most advanced stage of any non-signatory. Westminster has secured commitments from members such as Japan to support its bid for membership as part of bilateral trade agreements, and UK trade minister Liz Truss has stated the country expects to make an application to join in the near future.

Speaking at a City & Financial Global conference in January, Ms Truss described the CPTPP as "one of the world’s most dynamic trading areas", adding: "We will shortly submit our formal request to join this free trade area."

For the UK, CPTPP membership would offer better access to lucrative markets in Asia, though at present, trade with member countries only accounts for a small fraction of the nation's trade. But with the potential for further expansion with the likes of China, it could offer significant gains, as well as a route into new markets for existing members.

United States

As for the US, which withdrew from the TPP under the administration of Donald Trump, the outlook is less certain. There have been some bipartisan moves to support reentry into the new CPTPP, with even president Trump signalling the US could be persuaded back to the table if the conditions were right. 

However, new president Joe Biden still faces opposition from both sides of the aisle in Washington, with progressive members of his own Democrat party wary of its potential effect on areas such as manufacturing in the US.

It seems that, even if conditions are right, there is still a long way to go before the US could be considered as a member of the CPTPP and be in a position to win enough domestic support for the deal, even if the Biden administration looks favorably on it.

Speaking to CNBC recently, former US diplomat Dane Chamorro said: "I think that era has passed the United States for some period of time. I think it's a really hard sell, if you're a congressman or senator, regardless of what party you’re from, it’s really tough".

Other new members?

These are not the only nations to express interest in joining the CPTPP. South Korea, for instance, has indicated in January it will pursue membership of the group, which would add another developed country with a strong high-tech manufacturing industry to the bloc.

Finance minister Hong Nam-Ki said: "This is a must-go path to advance the domestic trade system to meet international norms and to respond to the acceleration of the post-COVID-19 digital economy."

Other nations said to be considering applications include Thailand, the Philippines and Colombia. While many of these may be several years away, the CPTPP could eventually become the world's largest-ever free trade area, with the potential to cover up to 90 per cent of global GDP.