The departure of Britain from the European Union will almost certainly put pressure on the British economy and cause additional problems for the poor, especially for young people, because living costs will likely rise and money from Brussels will dry up, according to a joint assessment by seven leading children’s charities.

The rapporteur was particularly scathing about bungled efforts to streamline the way welfare payments are made to individual recipients after delays in a shift to a new system, known as Universal Credit, led thousands of people to fall into poverty.

“The introduction of Universal Credit and significant reductions in the amount of and eligibility for important forms of support have undermined the capacity of benefits to loosen the grip of poverty,” Mr. Alston’s statement said.

His remarks came at the end of a tour of nine towns and cities in Britain, where the rapporteur’s team met with hundreds of welfare recipients, anti-poverty campaigners, officials and politicians. His findings were also based on analysis of nearly 300 written submissions from academics and members of the public.

Although poverty rapporteurs typically spend most of their time in the developing world, Mr. Alston said it was important to visit Britain, the world’s fifth-richest country, because its policies were a litmus test for the politics of austerity.

“The U.K. was a world leader in social security after World War II, it was a world leader on privatization on a large scale, and it is a world leader right now in self-imposed austerity,” he said in an interview earlier in his visit. “And so it is an important case study to better understand the implications of an austerity approach.”

Koldo Casla, a poverty researcher who submitted evidence to Mr. Alston during his visit, welcomed his conclusions, in particular the way he drew a direct line between government policy and British poverty.

“We must hold those responsible accountable,” said Dr. Casla, policy director at Just Fair, a poverty campaign group. “These last two weeks, Alston has been an invaluable loudspeaker for hundreds of people that are too often paid very little attention to.”