Liam Fox's international trade department is facing questions over procurement rules after demanding government contractors are a 'cultural fit' with Brexit plans.
Lib Dem leader Tim Farron has written to civil service chief Jeremy Heywood to demand an investigation and an apology from ministers.
Adverts from the Department for International Trade said companies must “be committed to the best possible outcome for the United Kingdom following its departure from the European Union.”
But Farron has questioned whether the wording breaks the rules, which include strict provisions on non-discrimination, free movement of goods, freedom to provide services and freedom of establishment.
“Public sector procurement is subject to a legal framework which encourages free and open competition and value for money, in line with internationally and nationally agreed obligations and regulations,” Farron said.
“I believe that these clear rules have been broken.”
Farron is demanding the government drop the demands for Brexit support, launch an investigation and apologise.
A DIT spokeswoman declined to comment on Farron's letter specifically, but denied that firms were being asked if they "back Brexit", adding: "It's natural for any Government department to want to work with firms that are positive about and focused on whatever task it is they are being asked to do."
It comes after foreign secretary Boris Johnson yesterday condemned “droning and moaning” about the risks of the EU, while Deloitte agreed to pull out of bidding for government contracts for six months after a damning report it had written about Downing Street's plans was leaked.