The United Nations expert whose warning of deepening poverty in Britain was this week dismissed as “barely believable” by ministers, has said the government’s denial is as worrying as the poverty itself.

Philip Alston, the UN rapporteur on extreme poverty and human rights, published his final report on the state of Britain on Wednesday. In it he accused the government of the “systematic immiseration of a significant part of the British population”. Ministers responded that it was “a completely inaccurate picture of our approach to tackling poverty” and instead claimed the UK was among the happiest countries in the world.

Alston, an eminent New York-based human rights lawyer, said the government response amounted to “a total denial of a set of uncontested facts” and that when he first read its public comment “I thought it might actually be a spoof”.

He said he feared it showed ministers were not willing to debate official figures that showed 14 million people were living in relative poverty and therefore consider what he believes are essential changes to the welfare system.

“The statement is as troubling as the situation,” he said. “There is nothing that indicates any willingness to debate over issues which have generated endless very detailed, totally reputable reports across the political spectrum in the UK. All of these are dismissed.”

Alston’s report compared Conservative policies to the creation of Victorian workhouses. Amber Rudd, the work and pensions secretary, said she felt it was politically biased and alleged that Alston did not do enough research, only visiting the UK for 11 days.

The government said it would complain to the United Nations and the UK’s ambassador in Geneva is understood to have this week requested a meeting with the UN high commissioner on human rights over the matter.

When Alston said the Department for Work and Pensions had created “a digital and sanitised version of the 19th-century workhouse”, some commentators said he had gone too far. The historian Dominic Sandbrook wrote in the Daily Mail that it was “simply ridiculous” and “an insult to our national intelligence”.

But far from backing down, Alston, who describes his politics as progressive and left-of-centre, has pushed his argument harder.

“I think breaking rocks has some similarity to the 35 hours of job search [required per week to receive universal credit] for people who have been out of work for months or years,” he said. “They have to go through the motions but it is completely useless. That seems to me to be very similar to the approach in the old-style workhouse. The underlying mentality is that we are going to make the place sufficiently unpleasant that you really won’t want to be here.”

In response to his report, the government cited research showing the UK was one of the happiest places in the world to live. It appeared to be a study that placed the UK 15th behind 12 other European countries including Germany, Ireland, the Netherlands and the whole of Scandinavia.

“That takes the denial on the part of the government to new heights,” Alston said. “The government is proceeding as if the problems I have reported don’t exist. Is it the case that 14 million people do not live in poverty? Do they contest the child poverty predictions? That is what it seems to be.”

Alston may have overstated one of the most startling statistics in his report, that “close to 40% of children are expected to be living in poverty by 2021”.

It was based on a forecast rise from 30% in 2016 to 37% in 2021 made 18 months ago by the Institute of Fiscal Studies, which now says it is likely to be an overestimate. The latest official figures, published in March, revealed the proportion of children in relative poverty has remained flat at 30% to 2018.

Alston said the figures were still bad: “We go down from 40% to one-third of British children. Is that a good result?”

Alston, a law professor at New York University, has been the UN poverty rapporteur since 2014 and has carried out investigations in countries including the US, Ghana, Saudi Arabia, China and Chile. He said governments normally responded with a detailed analysis or refutation of his reports but that had not yet come from the UK. Laos, which he investigated earlier this year, had already filed a detailed 20-page response.

In his reports he deliberately avoids “the arid and evasive language of diplomacy”, and that often leads him into political conflict.

He said it was a disgrace that the UN did not accept full responsibility for the cholera outbreak its workers imported to Haiti after the 2010 earthquake. In 2018, he accused the Trump administration of being driven by “contempt, and sometimes even by hatred for the poor” while “bringing in massive tax breaks for corporations and the very wealthy”. Trump’s ambassador to the UN at the time, Nikki Haley, accused him of bias and being “misleading and politically motivated”.

Alston, 69, a married father of four, studied law at Melbourne University and moved to Berkeley in California for his doctorate in the late 1970s. He has a long association with the UN, including as an official in Geneva for six years and chairing its committee on economic, social and cultural rights for eight years. As UN special rapporteur on extrajudicial, summary or arbitrary executions for another six years, he travelled to countries including Sri Lanka, Nigeria, Afghanistan and Ecuador.

He is well-regarded by colleagues who in January sprung on him a surprise academic conference. He arrived for what he expected to be a routine event only to find a two-day seminar dedicated to his human rights work. Instead of party games and music, scholars, who had been planning the event in secret for two years, gave papers and panel discussions at what they dubbed “Philipfest”.