LONDON (AP) — Trade policy experts are warning U.K. politicians campaigning in next month’s election that they are promising unattainable trade benefits and setting “arbitrary deadlines’’ for sealing a deal with the European Union after Brexit.

A dozen signatories to a letter published in the Daily Telegraph Wednesday urged leaders to supplant rhetoric with fact-based discussions about Britain’s future outside the EU trading bloc - or risk deals the public won’t support.

The letter says, “engines are being revved towards striking new (free trade agreements) by arbitrary deadlines with little sense of what we actually want to achieve.’’

The bipartisan letter, whose signatories include Alan Winters of the U.K. Trade Observatory and David Tinline, a former senior adviser to the World Trade Organization’s director-general, criticizes politicians on both sides of the Brexit issue.