A joint document on Britain’s post-Brexit relationship with the EU fails to offer any hope of frictionless trade, said to be vital to the British economy, but provides Theresa May with arguments to bolster her hopes of selling the deal to Brexiters in parliament.

A leaked 26-page political declaration, to be approved by EU leaders at a Brexit summit on Sunday, paints a picture of the future relationship that differs substantially from the proposals made by the prime minister at Chequers in the summer.

Brexit: leaked political declaration in full Read more

May is expected to give a statement on the declaration to the House of Commons on Thursday.

Speaking outside Downing Street after briefing the cabinet by phone, May said the agreement “delivers on the vote of the referendum”, and urged MPs to support it.

She said: “The British people want this to be settled. They want a good deal that sets us on course for a brighter future. That deal is within our grasp, and I am determined to deliver it.”

According to the declaration the two sides “envisage having a trading relationship on goods that is as close as possible”, but the EU and the UK would be separate markets with inevitable barriers to trade, and there is no reference to a common rulebook.

The document does reassert the plan for both sides to “build and improve on the single customs territory” already negotiated in the withdrawal agreement.

The UK has accepted in the withdrawal agreement, approved by both sides, that the country will stay in a customs union with the EU, should a trade deal that can avoid a hard border on the island of Ireland not be ready by the end of the transition period.

The text reiterates the longstanding EU red line that British access to European markets will only be offered on the basis of “open and fair competition”. To gain a close economic relationship, the UK would have to respect EU standards on competition, tax, environment, as well as social and employment protection.

But in a major sop to the Brexiters in her party, the prime minister has successfully argued for the inclusion of language that she might use to convince her critics that the customs union backstop may not be the only solution in the long term.

Quick guide Brexit and backstops: an explainer Show Hide A backstop is required to ensure there is no hard border in Ireland if a comprehensive free trade deal cannot be signed before the end of 2020. Theresa May has proposed to the EU that the whole of the UK would remain in the customs union after Brexit, but Brussels has said it needs more time to evaluate the proposal. As a result, the EU insists on having its own backstop - the backstop to the backstop - which would mean Northern Ireland would remain in the single market and customs union in the absence of a free trade deal, prompting fierce objections from Conservative hard Brexiters and the DUP, which props up her government. That prompted May to propose a country-wide alternative in which the whole of the UK would remain in parts of the customs union after Brexit. “The EU still requires a ‘backstop to the backstop’ – effectively an insurance policy for the insurance policy. And they want this to be the Northern Ireland-only solution that they had previously proposed,” May told MPs. Raising the stakes, the prime minister said the EU’s insistence amounted to a threat to the constitution of the UK: “We have been clear that we cannot agree to anything that threatens the integrity of our United Kingdom,” she added.

The document claims that “facilitative arrangements and technologies will also be considered in developing any alternative arrangements for ensuring the absence of a hard border on the island of Ireland on a permanent footing”.

The pro-Brexit European Research Group, chaired by the Conservative MP Jacob Rees-Mogg, has expressed its vehement opposition to a customs union, and lobbied hard for such an alternative plan, known as “maximum facilitation”, to avoiding a customs border being drawn in the Irish Sea.

Following the leak, an ERG source said: “The political declaration is not legally binding, vague, aspirational and little more than a smokescreen to cover up the fact that the permanent relationship is the customs union backstop.

“The withdrawal agreement gives away £39bn for no guaranteed free trade agreement in return. What it does guarantee is the UK would be stuck as a vassal state accepting EU laws and trade policy unless the EU decides to release us.”

The document is otherwise littered with vague aspirational language which fail to secure commitments from the EU on key British demands.

On financial services, it merely commits the EU to “endeavouring” to assess before the end of June 2020 whether UK firms could continue to operate in Europe.

There is no agreement that the UK can remain in key agencies such as the European Medicines Agency and the European Aviation Safety Agency, only offering to “explore the possibility of cooperation”.

The document raises cooperation with Europol, and EU security databases, without spelling out how the country’s police will be able to protect British citizens after Brexit.

On the UK’s involvement with the Galileo satellite project, to which the British government has been a major funder, the political declaration states: “The parties should consider appropriate arrangements for cooperation on space.”

Donald Tusk, the European council president, tweeted: “I have just sent to EU27 a draft political declaration on the future relationship between EU and UK. The commission president has informed me that it has been agreed at negotiators’ level and agreed in principle at political level, subject to the endorsement of the leaders.”

A spokesman for the European commission said the “ball is in the member states’ courts” to agree on the deal negotiated between Brussels and Downing Street, but that further work was ongoing on the contentious issues of Gibraltar and fisheries.

Spain is demanding that any future trade deal should only apply to Gibraltar, a disputed territory, if Madrid gives its consent.

In her statement in Downing Street, May expressed confidence that the issue could be solved, saying: “Last night I spoke to the Spanish prime minister, Pedro Sánchez, and I am confident that on Sunday we’ll be able to agree a deal that delivers for the whole UK family, including Gibraltar.”

No 10 feels that the text of the political declaration has improved considerably in the last few days, and is particularly satisfied with the language on “alternative arrangements”. It was felt by the UK that it made sense to agree the document promptly to lock in the gains that had been made.

The document further commits both sides to coming to an agreement six months before the end of the 21-month transition period after Brexit on a fisheries deal on “access to waters and quota shares”.

France was particularly keen for the political declaration to link the UK’s willingness to stay close to the current arrangement with the bloc’s opening up of its markets to British exports.

The leaked draft text, obtained by the Guardian, outlines the basis for future relations between the UK and EU, spanning the economy, security, foreign policy and how to manage possible disputes.

Trade talks, which the Brexit-supporting international trade secretary, Liam Fox, has said will be “the easiest in human history”, are due to begin in April 2019.

In the text, both sides agree to start formal negotiations “as soon as possible” after the UK leaves the EU on 29 March 2019, with the aim of having a deal enter into force by the end of 2020.

The timing sets the bar high, suggesting that negotiators on both sides are confident they can strike a deal far more quickly than the seven-year EU-Canada talks. It also shows both sides want to avoid the Irish backstop, as well as an extended transition that would leave the UK following EU rules without any say.

The two sides would meet through “a high-level conference” every six months to assess progress. In a win for May, the document proposes regular EU-UK summits, one of the prime minister’s Chequers proposals.