Sir Keir threatened to scupper the Prime Minister’s Brexit schedule when the European Union withdrawal legislation returns to the House of Commons tomorrow for a marathon debate.
He is expected to trigger a vote calling for a time-limited transitional deal on the same terms the UK currently has.
This would mean staying in the single market and customs union for a period of about two years after the UK has quit the Brussels bloc as well as the controversial European Court of Justice.
And the Labour shadow Brexit secretary hopes splits in the Conservative party can help secure him a victory, claiming the PM does not have the “authority” to deliver her own proposed transition deal.
In an open letter to Theresa May, sent to reporters before the prime minister, Keir Starmer said there were enough opponents of her approach within her own party to scuttle her proposed two-year transition period.
And Sir Keir said Labour would put forward amendments to the EU (Withdrawal) Bill to authorise its own transition plan.
He wrote: “It has become increasingly clear that you alone do not have the authority to deliver a transitional deal with Europe and to take the necessary steps to protect jobs and the economy.”
He noted that, in her recent Florence speech, Theresa May proposed “an implementation period of around two years” after March 2019 that “should continue on current terms” and would leave the UK in “the existing structure of EU rules and regulations”.
But Sir Keir says that, according to the EU’s Article 50 guidelines, “that can only mean abiding by the common rules of the single market and a customs union, including continued jurisdiction of the European Court of Justice (ECJ) during this time-limited period” – something Mrs May has be vocal against.
The Labour MP argued that, given the EU’s guidelines, the government’s position on transitional arrangements is unclear as the current withdrawal bill draft cannot deliver a time-limited transitional period on the same terms because it brings an immediate end to any role for the ECJ.
And he claimed Mrs May’s plan does not have enough support.
He wrote: “We have counted 14 Conservative MPs, including the foreign secretary, who have ruled out or rejected a transitional deal on the terms you have outlined. That is more MPs than the government’s working majority.”
The shadow Brexit Secretary added the United Kingdom could seek to remain under the ECJ’s jurisdiction in the “longer term” to retain membership of EU-managed agencies.
He said “I think the jurisdiction will have to change but to rule it out altogether does mean effectively that it’s very difficult to stay in some important agencies that we are currently part of.
“The Prime Minister simply said ‘I’ll have nothing whatsoever to do with the ECJ in any circumstance’ rather than saying ‘look, the role may have to change. Let’s be grown up and sensible about what it might be.’
“If there are places where the role of the court will make it better or easier for us to have the right relationship in the future, I’d be up to that.”