The government has been accused of abandoning its hardline position on EU migrants’ benefits, overturning decades of British policy and a key demand of David Cameron’s renegotiation of the UK’s status within the EU.

The British will vote on Wednesday in favour of an update to the EU’s social security regulation that would allow EU job seekers to export unemployment benefit for six months, compared with three months now.

Job seekers classified as “frontier workers” – because they live in one country and work in another – could claim benefits from the member state they worked in for up to 15 months, compared with a current limit of six months.

The UK would have to apply this law if parliament opted for a Norway-style soft Brexit that means permanent rule-taker status. The Norway option, sometimes rebranded as “common market 2.0”, means keeping free movement of people, currently the reddest of Theresa May’s red lines.

The British support for the law, which is expected to be confirmed at a meeting of EU ambassadors on Wednesday, has stunned traditional northern European allies that take a hard-nosed look at public finances. Austria, Belgium, Denmark, Germany, Luxembourg and the Netherlands are unhappy with the plans, but have been unable to muster a blocking minority without British support.

“We have fought on this file shoulder-to-shoulder for 40 years,” said an EU diplomat from this “like-minded” group. “You [the UK] were even more adamant than we were. And all of a sudden you can accept a prolongation from six to 15 months [for frontier workers]. It’s mind-boggling that you want these bad rules,” the diplomat said.

“You could have been the ringleader of our like-minded group and that’s what makes it so awkward that you are in the company of Italy and Spain and people who love free money.”

France, Spain, Italy, Portugal and central eastern European countries are supporting a compromise that was hammered out last week between the EU’s council of ministers and the European parliament.

British officials said their support for the regulation was based on genuine policy reasons. The UK has far fewer “frontier workers” than Austria, Germany or Luxembourg, where 180,000 people commute each day to the Grand Duchy for work.

The controversy has been focused on the export of benefits. The new rules would mean, for example, that an unemployed French accountant who used to work in Luxembourg could claim Luxembourg’s unemployment benefits for up to 15 months while living in France.

EU diplomats argue that Brexit has upturned the UK’s traditional priorities. Cameron fought the 2015 election with a manifesto promise to stop EU migrants claiming jobseeking benefits. One result of his 2016 EU renegotiation was that newly arrived EU workers would have been restricted from claiming non-contributory, in-work benefits for four years. The UK would have been able to apply this rule for seven years if the remain campaign had won the 2016 referendum.

“[The British] are totally against their traditional position,” a second EU diplomat said. “It’s a pretty strong example of how overwhelmed they are by Brexit. It’s just ‘what the hell, let’s get this through.’”

The UK’s imminent departure means the government has taken a backseat on some EU policies, but allies are unconvinced by this reasoning.

The second diplomat said: “When you look at the situation in parliament in Great Britain, I don’t understand this argument, because we don’t know if they will leave, when they will leave. We are only at stage one of this whole [Brexit] process. It’s so unclear what happens with the future relationship.”

Others are more philosophical. “For some time we have been used to the UK not taking any major decision any more,” a third EU diplomat said. “They weren’t such a strong force in these last months.”

A UK government spokesman said: “The UK supports these reforms overall – along with many other member states – which we have helped develop and which will simplify how out of work benefits are paid to EU migrants living in other countries.

“The UK’s eligibility rules are stringent and many potential applicants in or from the EU will not qualify unless they have worked here for a significant period. Only around 100 people exported jobseeker’s allowance from the UK to another member state last year.”