Keir Starmer will use a speech at the party's annual conference in the southern English town of Brighton to press home a shift in policy on Brexit which sees Britain keeping full access to the EU's single market during a transition period.
After months of sticking to a position little different from the Conservative Party's pursuit of a clean break with the EU, Labour changed tack last month by suggesting the four-year transition period to smooth Britain's departure in March 2019.
But while challenging Prime Minister Theresa May's vision for Brexit, the shift has also put a spotlight on the divisions in the Labour Party. Some lawmakers are saying leader Jeremy Corbyn should go further and say, if in power, Labour would stay in the single market for the long term.
"Labour are now the grown-ups in the room," Starmer will say to hundreds of Labour supporters on Monday, according to excerpts of his speech. "We stand ready to take charge of the negotiations. Not acting for narrow political gain. But in the national interest."
Labour is riding high since winning more seats than it was expected to in a June election, when May's attempt to win a stronger hand in the Brexit negotiations failed and lost her Conservatives their majority.
It has capitalised on divisions in May's team of ministers by offering what some call a "softer" vision for Brexit, saying that Britain has run out of time, largely because of that election, to negotiate a bespoke arrangement for a transition.
"The way the Tories (Conservatives) are handling Brexit tells you a lot about their competence - or should I say incompetence," Starmer will say, describing Labour's approach as one that will "puts jobs and the economy first".
But Labour is unlikely to move much further on its Brexit position at this week's conference, with Corbyn saying he would listen to those who are calling for Britain to stay in the single market and customs union for the long term.
A natural eurosceptic, Corbyn told reporters: "I would also say that we need to look very carefully at the terms of any trade relationship because at the moment we're part of the single market, obviously."
"That has within it restrictions in state aid and state spending."
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