Britain's largest banks have written an open letter to Prime Minister Theresa May calling on the government to ensure financial regulation remains aligned with the European Union under a BREXIT deal, the Guardian reports. Amid rising concern that big European banks will scale back their operations in London after BREXIT occurs in 2019, the banking industry body, UK Finance, warned that British GDP could take a big hit.  "A high degree of mutual cross-border market access is fundamental to the continued success of our financial services sector - and to the success of the economies and citizens which our sector serves in the UK and the EU 27," UK Finance told May. US banks such as Goldman Sachs are already considering proposals to move their European headquarters to Frankfurt.