Tim Roache wrote to Theresa May to demand a report in the next budget
The GMB said the Government should stop "hiding" information drawn up for ministers on several models of Brexit, examining how different sectors of the UK economy would be affected.
In a letter to the Prime Minister, GMB general secretary Tim Roache said: "By not publishing the reports, your departments are keeping all of those with an interest in the success of UK industries in the dark.
"Keeping this information from the public is preventing us from working together to navigate the challenges, mitigate the risks and take advantage of any opportunities that may arise as we leave the European Union.
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"The public also want to see the risk reports published. In March 2017, GMB published opinion polling which showed more than twice as many members of the public want to see these reports published than those who do not."
Mr Roache said the Government should engage with trade unions and employers to safeguard jobs, adding: "Refusal to publish will only increase suspicion that the Government is making decisions relating to Brexit against the economic interests of working people."
The union has been calling on the Government to publish its results for months due to fears of a “hard Brexit”.
In an article on its website from July, it claims the OBR warns of a £100billion cost from weakened productivity growth alone.
It added: “This means lower wages and worse public services and is exactly why we need a proper industrial strategy that puts jobs and investment first, as GMB has called for.”
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Downing Street said “all contingencies” will be discussed by cabinet members in the regular Number 10 session.
That will include what happens if there is no deal, alongside the PM’s hopes for progress on a trade agreement.
It comes amid mounting pressure from her own party to prepare for the prospect of failing to reach an acceptable agreement with Brussels.
GMB has been calling for the Government to produce a report for months
A spokesman for Mrs May said: “It is looking at Brexit preparations for all contingencies.
“That obviously does include 'no deal', but as the Prime Minister has said on many occasions, we are working towards getting a deal and we are confident that we will do so.
David Davis is expected to lead the Cabinet discussions
"Cabinet has been updated on Brexit preparations regularly. This is another opportunity to do so.”
Asked whether ministers would discuss the possibility of Brexit not happening, he said: "We have been very clear that we are leaving the European Union."