Banks, accountants and law firms that facilitate offshore tax schemes face a Europe-wide crackdown, according to a leak of draft legislation.

Brussels will publish proposals this Wednesday to force financial intermediaries to automatically disclose any new cross-border tax schemes offered to clients. Those designing and promoting aggressive avoidance structures will have five working days to file details with their local tax authority, according to a leaked version of the proposals, drawn up by the European commission.

The clock will begin ticking as soon as the scheme has become available to a client. Where there are several intermediaries in the chain, one will be made to take responsibility for disclosure. And where all intermediaries in the chain are based outside European member states, the obligation to disclose will fall to the client.

“The ultimate objective”, according to the commission, “is to design a mechanism … that will dissuade intermediaries from designing and marketing such arrangements.”

The new rules will come into force in 2019 and are aimed at cross-border schemes that involve more than one country, so long as one of the jurisdictions involved is within Europe.

Since 2004, UK statute books have had legislation forcing those who market tax schemes to report them to Revenue & Customs. Portugal and Ireland have similar rules. However, the commission’s proposals would further tighten the screw on British-based intermediaries.

This is because all European member states will be obliged to share with each other, every three months, details of the tax schemes disclosed. A central directory of avoidance schemes will be created, to which all member states will have access.

It is possible the regulations will never be adopted by the UK. However, if Britain negotiates to remain part of the single market, it would be subject to the same tax and financial regulation as full members of the union.

Research shows that globally, the majority of intermediaries are based in Hong Kong, the UK and the United States. A study of the ICIJ Offshore Leaks database, which contains data from the Panama Papers and previous leaks, identified 140 intermediaries linked to offshore entities. Nearly 90% of them have an office, a subsidiary or an affiliate in Europe.

The most active facilitators were the Swiss banks UBS and Credit Suisse, which were linked to 24,500 offshore entities between them, according to the report, which was commissioned by the Green and European Free Alliance groups in the European parliament.

Trident Corporate Services, which has offices in London’s Portland Place and was linked to 8,500 offshore entities, is the third largest and the first in a string of middlemen whose names are largely unknown outside the world of offshore companies.

“If we go for a softer Brexit, as now seems more likely, these rules would apply in the UK,” said a Green MEP for south-west England and Gibraltar, Molly Scott Cato. “We call on member states to adopt the proposal as soon as possible and to scale up resources in their tax administrations.”