Separating the Brexit divorce deal from the framework for the next stage of the relationship with Brussels would leave MPs “completely blind” about the future, Sir Keir Starmer has said.
The shadow Brexit secretary said such a move by Theresa May would be “desperate”.
The Government has signalled that MPs will be asked to vote on some form of Brexit motion in the Commons on Friday, fuelling speculation that Mrs May will ask them to approve the Withdrawal Agreement reached last November, but not the Political Declaration setting out plans for a future trade and security relationship with the EU.
It comes after the Prime Minister told Tory MPs on Wednesday she would step down to let a successor take charge of the next phase of Brexit if her deal was passed.
Sir Keir, speaking to the Press Association at the British Chambers of Commerce conference in London, said Labour would reject the deal on those terms.
He said: “As the EU has made clear and as the Prime Minister has made clear, the Withdrawal Agreement and the Political Declaration are part of the same negotiated package. You can’t have one without the other.
“The problem going ahead with just the Withdrawal Agreement is it’s a completely blind Brexit. It tells you nothing about where you are heading.
“That’s been made a lot worse by Theresa May saying yesterday she is going to step down as Prime Minister. So we don’t even know who is going to take over and where they are going to take this.
“You can’t separate them, this isn’t going to work. It’s a desperate measure.”
Sir Keir said the indicative votes process begun on Wednesday and set to continue on Monday could reveal what sort of Brexit deal could command a Commons majority.
And he suggested that an extension to the Article 50 process, delaying Brexit, should be sought without a fixed deadline to avoid another cliff-edge.
“We said we want an extension which is as short as possible. But what I am concerned about is that we have a purpose, that we know why it is that we want an extension and what we are trying to achieve.
“I think simply setting another clock running would just repeat the mistakes of the last two years.”
He added: “We need to know what the majority is, we need to agree a purpose and then we will know for how long we need an extension. That will be a matter of discussion with the EU.”
Sir Keir acknowledged Labour’s internal divisions over Brexit, which saw Melanie Onn quit as shadow housing minister to oppose a “confirmatory” referendum on any deal which gets through Parliament.
But he said: “At this stage, whatever scrapes through needs to be settled by a confirmation ballot.”
He suggested that on Monday the “three or four options that have the highest votes” should be considered again in some form.
Based on Wednesday’s results, those options could include a customs union, a second referendum, Labour’s plan and the Common Market 2.0 proposal for a Norway-style deal alongside a customs arrangement.