Theresa May’s Brexit strategy has been thrown into new doubt as a former head of the government’s legal services ridicules the prime minister’s claim that the UK can break free of all European laws while continuing to reap the economic benefits of the EU’s single market.

Sir Paul Jenkins, who was the government’s most senior legal official for eight years until 2014, told the Observer that the prime minister’s policy on the legal implications of Brexit was “foolish”. He insisted that if the UK wants to retain close links with the single market and customs union it will have no option but to observe EU law “in all but name”.

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The comments – backed by several other leading experts on EU law – cast serious doubt on the central plank of the government’s latest thinking on Brexit, with less than two weeks to go before Brexit secretary David Davis enters a crucial phase of talks on the exit plans in Brussels.

After a summer of cabinet wrangling, ministers last week published the first of a series of papers on their Brexit negotiating position in an attempt to clarify the UK’s stance. They made clear that while the UK will leave the customs union and single market, it will seek to stay as closely linked as possible to both, and reject any hard border arrangement with the Republic of Ireland.

This week more papers will be released, including one spelling out how legal disputes could be resolved between the UK and EU once the European court of justice (ECJ) no longer has direct jurisdiction in the UK. Leaving the ECJ has been one of the totemic aims of Eurosceptics and any government U-turn on the issue would provoke an outcry among Tory Brexiters. May has repeatedly said that the UK will break free of the ECJ and leave its jurisdiction on the day of Brexit.

Q&A Why is Dublin opposed to the idea of a hard border? Show Hide Ireland’s taoiseach, Leo Varadkar, has been much more sceptical than the UK about the potential for avoiding border posts via virtual checks on importers. Whilst agreeing with British ministers and EU negotiators that it is inconceivable for there to be a return to a hard border with the north, Dublin argues that the best way for the UK to achieve this would be by permanently remaining in a customs union with the EU and seeking single market membership like Norway through the European Economic Area. The UK has conceded that some of this will be necessary in its interim phase after Brexit, but hopes clever technological solutions can allow it have looser economic links in the long run. Varadkar is not alone in being sceptical about whether such a cake-and-eat-it customs and trade strategy is viable.

UK and EU legal experts are becoming increasingly vocal in asserting that the prime minister’s policy is unrealistic and impossible to achieve. Jenkins, now an associate member of barristers Matrix Chambers, said: “If the UK is to be part of something close enough to a customs union or the single market to remove the need for hard borders, it will only work if the rules are identical to the EU’s own internal rules.

“Not only must they be the same but there must be consistent policing of those rules. If Theresa May’s red line means we cannot be tied to the ECJ, the Brexit treaty will need to provide a parallel policing system.

“That may be a new court but, in reality, any new court will have to follow what the ECJ says about the EU’s own rules, otherwise the new system won’t work. So, never mind Theresa May’s foolish red line; we will have the ECJ in all but name.”

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Writing in the Observer, Catherine Barnard, professor of EU law at Trinity College, Cambridge, also casts serious doubt on whether the UK can escape ECJ jurisdiction while staying close to the single market and customs union. Steve Peers of Essex University also says such an approach is “simplistic” and the judicial version of “have our cake and eat it too”.

The remarks highlight the increasing difficulties May and her ministers are having to confront as they try to meet the twin demands of business, which fears a hard Brexit and loss of access to EU markets, and Tory pro-Brexit MPs, who want a complete break from the EU.

In her keynote speech on Brexit in January, May was explicit about the need to break away from the ECJ and its judicial rulings, saying: “We will not have truly left the European Union if we are not in control of our own laws.”

She added: “Leaving the European Union will mean that our laws will be made in Westminster, Edinburgh, Cardiff and Belfast. And those laws will be interpreted by judges not in Luxembourg but in courts across this country.”

The government’s insistence that the UK must leave the jurisdiction of the ECJ after Brexit could also mean Northern Ireland loses tens of millions of pounds in funding for its peace and reconciliation programmes.

Open Britain, which campaigns against a hard Brexit, claims that unless the government gives ground, the EU’s Peace programme – which under its fourth round of funding covering the period 2014 to 2020 is set to receive a total of £208m from the European regional development fund – is in jeopardy. The programme, which aims to boost “cohesion between communities involved in the conflict in Northern Ireland” and “economic and social stability”, applies to Northern Ireland and the six border counties of the Republic.

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Leading constitutional expert Vernon Bogdanor, also writing in the Observer, says it is becoming clear that an eventual deal on Brexit can only be legitimised through a second referendum because the country and politicians are so divided on the issue.

An Opinium/Observer poll finds that 60% of people now back a transition deal to ensure a smooth exit from the EU, rather than a complete break in 2019. Some 37% of people believe there should be a second referendum on the eventual outcome of negotiations, while 49% are against it.

The findings come after chancellor Philip Hammond and international trade secretary Liam Fox, widely seen as opposed on the issue, made it clear in a joint article last week that the UK would need a transition period.