In speeches focusing on the importance of free trade, Fox has previously hinted that future customs union membership is unlikely.
MPs are set to debate parliamentary scrutiny of Brexit decision making later this afternoon, with backbenchers already winning concessions from Downing Street on oversight.
And now MPs including shadow Brexit secretary Keir Starmer, Treasury Select Committee member Rachel Reeves and Home Affairs committee member Chuka Umunna, have ramped up the pressure on the government by demanding fresh information.
“Before we sleepwalk out of the Customs Union, we believe it is imperative that you are much more up front with Parliament and the public about the consequences of this decision,” the letter states.
“Unless you can materially demonstrate that we will be better off economically by leaving the Customs Union, it can only be concluded that you are being disingenuous about hypothetical future trade benefits and in denial about the real costs of your plans.
“A fantasy land of future trade deals is fine for Conference speeches but big decisions such as this should only be taken with a hard-headed assessment of the impact in the real world.”
The Department for International Trade had not responded to request for comment at the time of publication.
The full list of MPs signing the letter
Rushanara Ali MP; Dave Anderson MP; Jonathan Ashworth MP; Ian Austin MP; Adrian Bailey MP; Roberta Blackman-Woods MP; Ben Bradshaw MP; Alan Campbell MP; Jenny Chapman MP; Ann Clwyd MP; Vernon Coaker MP; Ann Coffey MP; Stella Creasy MP; Alex Cunningham MP; Nic Dakin MP; Stephen Doughty MP; Jack Dromey MP; Julie Elliott MP; Bill Esterson MP; Paul Farrelly MP; Mike Gapes MP; Helen Goodman MP; Kate Green MP; Andrew Gwynne MP; David Hanson MP; Sharon Hodgson MP; George Howarth MP; Tristram Hunt MP; Alan Johnson MP; Diana Johnson MP; Graham Jones MP; Mike Kane MP; Liz Kendall MP; Peter Kyle MP; Shabana Mahmood MP; Kerry McCarthy MP; Pat McFadden MP; Ian Mearns MP; Ian Murray MP; Madeline Moon MP; Grahame Morris MP; Bridget Phillipson MP; Rachel Reeves MP; Virendra Sharma MP; Barry Sheerman MP; Ruth Smeeth MP; Nick Smith MP; Keir Starmer MP; Stephen Timms MP; Stephen Twigg MP; Chuka Umunna MP; Phil Wilson MP