Britain's financial industry slammed U.K. Prime Minister Theresa May's latest proposal for Brexit, with some calling it the worst outcome possible.

In some of their most forthright comments since the referendum vote in 2016, industry executives said Thursday that her plan for a looser partnership with the European Union in financial services will drive up costs and hurt the wider economy. They also said it would leave the country losing easy access to its biggest market while also being at the mercy of the EU's rules.

May is now leaning toward an agreement for the U.K. that will be similar to so-called equivalence — existing arrangements the bloc has with other countries, based on maintaining similar regulations, according to the long-awaited white paper on Brexit. As a result, U.K.-based banks would lose their unhindered access to EU markets.