Jeremy Corbyn has called key members of his shadow cabinet to an “away day” to re-examine the party’s policy and strategy on Brexit amid growing frustration in Labour ranks that it is failing to exploit mounting Tory turmoil over Europe.

Party sources confirmed to the Observer that the meeting, scheduled for early February, would look at adapting and developing Labour’s approach during “phase two” of the Brexit process.

The gathering – which will be seen as a response to unrest and the threat of rebellions by dozens of Labour MPs – will be held at a location “away from Westminster”, and will involve senior shadow cabinet members in policy areas most affected by the UK’s departure from the EU.

The news suggests Labour may soon announce a major shift in policy that would see it back permanent membership of some form of customs union with the EU after Brexit – opening a potentially decisive dividing line with Theresa’s May’s increasingly fractured government.

A senior figure aware of the meeting said: “There are several among those who will attend who want the party to move on the single market and customs union. But Jeremy is a lifelong eurosceptic and there is still opposition to doing so.

“The greatest pressure for change is from those who insist we must back permanent membership of a customs union with the EU after Brexit, not just a fudge position of backing it during a transition and leaving open what happens after, which we have at present.”

Those who have been asked to attend are understood to include members of the shadow cabinet Brexit subcommittee. They include the shadow chancellor, John McDonnell, the shadow Brexit secretary, Keir Starmer, the shadow foreign secretary, Emily Thornberry, and the shadow home secretary, Diane Abbott. Shadow ministers responsible for Northern Ireland, Scotland and Wales will also attend.

With Theresa May’s government increasingly split over Brexit, and the EU withdrawal bill heading into the House of Lords on Tuesday, where it is expected to be savaged by pro-Remain peers of all parties as well as crossbenchers, a growing number of Labour MPs and peers are pressing the leadership to open up clearer dividing lines with the Tories.

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Trouble is also brewing for the Labour leadership on other Brexit bills, including one covering trade, when legislation returns to the Commons next month. Several amendments backing customs union and single membership are expected to secure the backing of at least 60 Labour MPs. Pro-Remain Labour MP Chris Leslie said: “There is lot of pressure. The time for a fudged approach is over. Unless we change in the next few weeks we will find it is too late. We will end up with a hard Brexit of the kind Jacob Rees-Mogg is demanding.”

An Opinium poll for the Observer last weekend found that potential Labour supporters now back permanent membership of the single market and customs union by a margin of more than four to one.

Labour, Tory and Liberal Democrat peers, as well some crossbenchers, believe that the government could be defeated on a series of amendments that will be tabled by pro-Remain peers on the customs union and single market if Labour changes position to back permanent membership.

While a move to back permanent membership of some form of customs union now looks the most likely shift, Labour campaigners for Corbyn to endorse UK inclusion in the single market believe he remains opposed.

Last summer Labour announced it would support the UK staying a full member of both the single market and customs union during a post-Brexit transition deal of two to four years. This would mean continuing to accept EU rules on free movement of people and workers, being subject to European court of justice rulings, and paying into the EU budget during the transition.

After the transition was over Labour said it would try to negotiate as close an economic and trading relationship as possible with the EU that would deliver the “exact same benefits” as the UK currently enjoys. Permanent membership of the single market and a customs union was not ruled out.

Theresa May then moved Tory policy in a similar direction in her Florence speech in September when she effectively conceded that the UK would remain inside the EU’s economic union, accepting EU rules including free movement in return, during a two-year transition. As result the Tory and Labour positions have become closely aligned.