A report commissioned by US businesses in Europe has warned Theresa May ahead of article 50 negotiations that American investment in the UK, worth £487bn in 2015, has been largely based on the country’s EU membership and access to the single market.

The study commissioned by the American Chamber of Commerce to the EU (AmCham EU) has called for Britain to recognise the limitations of any free trade deal with the US alone, and warns of the dangers posed in building barriers to trade between the UK and the wider continent.

It says the importance of the European market to US firms in Britain means that “both the US and the UK have more to gain from achieving some agreement with the EU than simply with each other”. The report suggested the record £487bn US firms invested in the UK in 2015 had been imperilled by the UK government’s intention to withdraw from the single market.

“The United Kingdom is a critical economic partner for the United States, and the US should ensure that bilateral ties are strengthened, rather than disrupted, by Brexit,” the organisation representing American businesses in Europe says.

“But America’s significant commercial and financial presence in the UK has been premised in large part on UK membership in the European Union – the largest, wealthiest and most important foreign market in the world to US companies.

“For decades, the UK has served as a strategic gateway to the European Union for US firms and financial institutions. The primary motivation of many US companies to invest in the UK has not been to serve only the UK market but to gain access to the much bigger EU single market.”

Some within the Conservative party support the UK walking out of talks with the EU to seek preferable deals with the rest of the world if the European commission’s chief negotiator, Michel Barnier, insists on the UK settling an estimated €60bn exit bill before any UK-EU trade negotiations can begin.

However, the report for the AmCham EU, authored by the Centre for Transatlantic Relations, at Johns Hopkins says many US companies are based in the UK because of its role as “a gateway to the single market” and that during any trade negotiations with Donald Trump’s administration, the Americans will want to know “how open, wide and strong that gateway will be after Brexit”.

May and Trump have opened what has been called a “trade negotiation agreement” that identifies potential stumbling blocks and scopes what could be done before the UK leaves the EU. The UK is not allowed to open formal negotiations as long as it remains an EU member.

The report highlights the importance to US banks of access to the European market through so-called passporting rights, and warns of the consequences if the UK is unable to negotiate similar access for companies based in the UK. “Many of these US firms will probably choose another entry point to access the single market in the future,” the reports says.

“This will make a huge difference with regards to London’s role as a financial hub, may accelerate the rise of other European financial centres, for instance Frankfurt, and will reinforce US interest in strong and predictable financial services procedures with the EU.

“It will also affect the US approach to financial services in any US-UK arrangement.”

It is likely that the EU will offer the UK membership of an “equivalence” regime for financial services that extends limited access rights to non-EU countries, such as the US, on the basis that their regulatory regimes are similar. However, the chamber warns that this is “relatively new and somewhat inconsistent approach with rights that are weaker than those granted under full passporting”.

The report concludes that UK-EU trade deal is unlikely to be in place until about 2025.

“When Washington sets out to negotiate a formal bilateral deal with the UK, it will want to understand the UK’s new WTO [World Trade Organisation] commitments and the nature of UK-EU transitional arrangements following Brexit, as well as London’s end goals with regard to a deal with the UK’s largest trade partner, the EU,” the report says. “This will all take time.”