Theresa May must “face down” hardline Brexiteers in her own Government who risk taking Britain into a “disastrous and divisive” new reality following withdrawal from the EU, a senior Labour figure has said.

Sir Keir Starmer said he anticipated splits emerging within the Conservatives over the two-year negotiation period, which could result in the Prime Minister being forced to go back and renegotiate if she presents Parliament with a bad deal.

The shadow Brexit secretary urged Mrs May to be more “open and collegiate” about the withdrawal process, warning that her guarded approach and her efforts to shut down dialogue were “unhealthy” for the prospects of a positive outcome.

Speaking at Chatham House, Sir Keir called on the PM to prioritise the agreement of a “transitional arrangement” to cover the period after Brexit until a good trade deal can be forged.

Rejecting her argument that a comprehensive free trade agreement can be concluded within two years and that “no deal is better than a bad deal”, he said exiting without a deal would be “the worst of all possible outcomes” and that “the right deal is better than a quick deal”.

Sir Keir’s warnings came two days ahead of the historic Article 50 letter formally triggering the UK’s withdrawal, as he hardened Labour’s position by setting out six tests for the deal Mrs May brings back from Brussels.

To win Labour’s support, he said the deal must:

Deliver the “exact same benefits” as the UK currently has as a member of the single market and customs union

Ensure a strong and collaborative future relationship with the EU;

Ensure the fair management of migration “in the interests of the economy and communities”

Defend rights and protections and “prevent a race to the bottom”

Protect national security and the UK’s capacity to tackle cross-border crime

Deliver for all regions and nations of the UK.

Theresa May will trigger Article 50 on March 29, what happens next? graphic

The shadow Brexit secretary said Mrs May had so far allowed the Government’s stance to be shaped by hardline Brexiteers who reject “the entire European social and economic model: employment rights, environmental protections, fair corporate tax rates, investment in public services such as the NHS”.

“This ideologically driven approach to Brexit would be disastrous and divisive,” said Sir Keir. “And it would stand as a roadblock to continued co-operation in the important fields of technology, research, medicine, security, science, arts and culture.

“The Prime Minister needs to face down these Brexiteers who would distance us from Europe.”

Mrs May’s official spokesman appeared to back away from Brexit Secretary David Davis’s suggestion in January that Britain can obtain a deal which delivers the “exact same benefits” as membership of the single market and European customs union.

“We are about to go into a negotiation,” said the spokesman. “We want the greatest possible access to the single market.”