LONDON -  Britain is attempting to "pull down the blinds" on its post-Brexit free trade talks with the United States after agreeing to keep key aspects of the negotiations confidential, critics said Thursday.

The two countries have set up a "trade and investment working group" to lay the groundwork for a future deal once Britain leaves the European Union in 2019.

In an exchange of letters between London and Washington released last week, the transatlantic allies agreed to mark information, papers and discussions held in the group as "sensitive" or "confidential".

They also committed to keeping these elements in the talks "in confidence" for four years following their conclusion.

Transparency advocates argue it will hinder media and public scrutiny of the discussions.

"It's going to be hard to convince people that they stand to benefit from Brexit trade deals if the first thing ministers try to do is cloak them in secrecy," said Doug Parr, Greenpeace UK's policy director.

"There are already widespread concerns about a US trade deal leading to weaker food safety and animal welfare standards -- things the UK public care deeply about.  "(International trade minister) Liam Fox's attempt to pull down the blinds on the trade talks will only stoke these fears, not allay them."

A spokesman for the Department for International Trade insisted it had been "completely open" about the talks.

"To support productive discussions, the working group does need to exchange some information in confidence, and these arrangements strictly adhere to Cabinet Office guidance on international exchanges of classified information," he said. The United States is Britain's largest trading partner outside the EU, accounting for a fifth of all exports, with combined trade worth £160 billion ($215 billion, 180 billion euros) a year.

But environmental campaigners and others have urged caution in agreeing any post-Brexit trade deal amid concerns over US animal welfare and food standards. Fox told the BBC last month that he wanted to avoid the mistakes of the Transatlantic Trade and Investment Partnership -- a proposed deal between the United States and the EU -- which has stalled following years of controversy and a Europe-wide public backlash.

Barry Gardiner, the opposition Labour Party's trade spokesman, said Fox's pledge for transparency "are empty words devoid of any meaning".

He called for enhanced parliamentary scrutiny.

"Trade agreements are not just about tariffs and barriers to trade -- they are about the standards, rules and regulation that determine what kind of society we live in," Gardiner said.