EU loyalist Peter Mandelson has promised “gruelling political trench warfare” as the Government prepares the European Union (Withdrawal) Bill for Parliament.

The so-called Prince of Darkness — who had to resign from the Blair Government twice before being gifted a job at the European Commission and made a Lord — issued his call to arms in an article for The Times Red Box.

“The most significant development of recent weeks has been Labour’s shift in stance on Single Market and Customs Union membership,” he crowed.

“The party is now calling on the government to keep Britain fully in the Single Market and Customs Union for a transitional period that lasts ‘as long as necessary’. Mr Starmer also said that continued single market and customs union membership after transition should remain on the table.”

Hard Brexit opponents, let your voices be heard, writes Lord Mandelson, in an exclusive comment for Red Box https://t.co/Fr0tc5nsnM pic.twitter.com/RFWlxen85n — Red Box (@timesredbox) September 4, 2017

The Young Communist League alumnus gloated that this U-turn means the Government will be walking into a Parliamentary “minefield”, with the Remain-supporting opposition parties sensing an opportunity to frustrate the Brexit process.

“This paves the way for serious, gruelling political trench warfare. It also means that only a small rebellion by Conservatives would be needed to defeat the government and force them to change course.”

"I think people will interpret [continued] membership of the Single Market as not respecting [the EU] referendum" concedes @johnmcdonnellMP. https://t.co/ODbiXrab8W — Jack Montgomery ن (@JackBMontgomery) June 11, 2017

Labour leader Jeremy Corbyn and chief lieutenant John McDonnell, who hail from Labour’s old, Eurosceptic rump, had previously appeared to commit to a full, clean Brexit from the Single Market and Customs Union, with Corbyn even going so far as to admit that the “wholesale importation of low-paid workers from central Europe” had been used to “destroy conditions” for British workers in industries like construction.

Shadow trade secretary Barry Gardiner even penned an article conceding that anything less than a clean Brexit would be “considered a con” by the voting public, and Shadow Brexit secretary Keir Starmer — who is now campaigning for transition arrangements which look very much like EU membership in name only, possibly permanently — said that “membership of the Single Market is incompatible with our clarity about the fact that Freedom of Movement rules have to change”.

The near-total reversal of this position suggests the Remain-supporting establishment in the Labour Party have been able to bring the Eurosceptic leadership under a degree of control.

Fo llow Jack Montgomery on Twitter: @JackBMontgomery