Chuka Umunna has called for transparency regarding the arrangement that persuaded Japanese car giant Nissan to commit to new investment in their Sunderland plant following the vote for Brexit.
The former Shadow Business Secretary piled pressure on the government when he wrote to Cabinet Secretary Sir Jeremy Heywood requesting the publication of assurances, "written or otherwise", which appear to have stabilised the company's warnings in recent weeks that it could relocate its Tyne and Wear operations to France unless it was protected from trade tariffs.
Downing Street has insisted Nissan had not been offered any form of "compensation package" and that ministers had simply assured the company of the Government's determination to secure the best possible deal after the UK withdrew.
Umunna wrote: "There has been speculation that the Government may have suggested that public money could be used to compensate Nissan, be it in direct aid or through more indirect means.
"Others have suggested that ministers may have given an indication about the UK's future role in relation to the customs union or single market.
"If either are true, they pose huge questions, most importantly what financial support was offered; whether any limit was placed on the public finance available; and whether the same assurances would be extended to other automotive-manufacturers and other sectors, including service sectors."
The MP for Streatham said that he welcomed the Government's aim to protect Britain's manufacturing base, but added: "It seems extraordinary that the Government would reveal elements of its negotiating strategy to multinational companies when it is at the same time doing its best to keep Parliament and the public in the dark."
Nissan's decision, which has been welcomed by Theresa May's Government as a "vote of confidence" in the UK economy, has secured 7,000 jobs at the plant and a possible 20,000 in the wider economy.
However, the Liberal Democrat former business secretary Sir Vince Cable and Conservative ex-business minister Anna Soubry have both said they believed Nissan would not have gone ahead with the investment unless it had been assured the UK would either remain in the customs union or there would be mitigation for any new tariffs it faced.