Michel Barnier conceded that securing backing for the provisional Brexit deal struck with the UK would be difficult as he unveiled a new customs union along with a joint pledge to keep the EU and UK in permanent regulatory lockstep.

As the EU’s chief negotiator revealed the withdrawal agreement and political declaration on the future deal in Brussels, he said the documents offered evidence of “decisive progress” in the talks.

Barnier told reporters in a press conference, however, that he recognised that the “path is still long” in getting agreement on both sides of the channel, and called on the UK parliament to “assume its responsibility”.

He said he felt neither a sense of self-satisfaction, nor smugness about Brexit, but only regret, adding that a withdrawal agreement was a necessary prelude to a future deal.

Barnier said the withdrawal agreement also ensured there would be a transition period of 21 months with the possibility of an extension by mutual agreement. The agreement actually says that the period could be extended to the year “20xx”.

In a boost to Theresa May, he said he did not expect a trade agreement to take as long as those negotiated with countries with different regulatory standards that are much further away.

The EU and UK have now agreed a 585-page text that covers citizens’ rights, the Brexit divorce bill and unlocked the thorny issue of avoiding a hard border on the island of Ireland.

Along with a joint customs union in the withdrawal agreement to ensure that a border was not drawn in the Irish Sea between Northern Ireland and Great Britain, a seven-page outline on the political declaration commits both sides to “combining deep regulatory and customs cooperation, building on the single customs territory provided for in the withdrawal agreement”.

Barnier said: “We’re drawing the basis for an ambitious partnership which we want, which is a free trade area based on regulatory and customs cooperation in depth and with a ‘level playing field’.

“Our objective is to abolish customs duties and quotas for all goods based on what we’re proposing in the withdrawal agreement, a single customs territory …

“There will be no tariffs on goods and indeed it is legitimate for that proposal to be flanked by a level playing field that’s a voluntary deliberate agreement between two parties who respect each other. This agreement offers a basis for future negotiations and that is in our common interest.”

The solution originally proposed by the EU to avoid a hard border with the Republic of Ireland under any circumstances would have, in effect, kept Northern Ireland in the EU’s customs union and single market as the rest of the UK departed.

“The backstop solution has evolved considerably from the original EU proposal from February this year”, Barnier said. “Over the last few weeks we have gone forward with the UK on the basis of their proposal. In the backstop protocol we agree to create an EU-UK single customs territory”.

Barnier added: “Northern Ireland will therefore remain in the same customs territory as the rest of the UK. In addition Northern Ireland will remain aligned to those rules in the single market that are essential for avoiding a hard border.”

Even before the documents were published the proposed Brexit agreement had been questioned by EU member states, however. France, Spain and Denmark all raised concerns with the European commission that the UK was being handed a customs union without sufficiently strong conditions.

At a meeting with Sabine Weyand, the EU deputy chief negotiator, ambassadors acting for the member states with the closest trading ties with the UK spoke of their need to ensure British companies could not undercut their own economies.

The Netherlands was also understood to be among those who harbour deep concerns that the British government was being handed too much in the last hectic weeks of the negotiations without sufficient guarantees.

EU diplomats said that the demands of the UK under the “level playing field” commitments could be thrown to the leaders at the extraordinary Brexit summit on 25 November to scrutinise and strengthen.

Member states further raised concerns that the commission had granted the UK a customs union offering tariff-free trade without getting assurances on the future fisheries deal.

The 27 member states had been hoping to use trade as leverage to secure something close to a continuation of the current arrangements for EU fishing fleets operating in British waters.

At a recent meeting Weyand had insisted that the UK would have to “swallow” a trade-off in which the UK was able to sell fish products into the European markets in return for EU fleets’ access to British seas. Barnier said he would insist in the trade talks that “a new agreement on fisheries” would be part of the deal.

The issue has, however, been pushed into the future trade negotiations rather than being included in the withdrawal agreement, alongside agreements on state aid, tax, environmental, social and labour regulations.

Ministers for the EU are to meet on Monday in Brussels. Sources suggested the most outspoken member states could make a point of airing their concerns as a prelude to changes at the special summit.

EU27 ambassadors met for nearly four hours on Wednesday night in the Europa building in Brussels at the same time as the British cabinet was meeting. Weyand informed the ambassadors that the UK had agreed to sign up to 27 pages of “level playing field commitments”.

The UK has accepted that it would have to stay in “dynamic alignment” with the EU on state aid rules, preventing anti-competitive subsidies for British businesses.

The UK would have put into British law three EU directives on tax: exchange of tax information, country-by-country reporting on investment firms and the EU’s taxation code of conduct

It has also committed that it would not regress on its social, environmental and labour regulations after Brexit.

Weyand insisted to the member states that the termination of the all-UK customs union would have to be a joint decision.