While Mr Miles has said in the past that migrants have a similar overall profile to other UK workers, he added that it was “much less clear that persistently high levels of net immigration to boost the labour force can generate sustained fiscal improvements”.
Mr Miles said: “Even if the new ONS projections of higher long run net immigration prove accurate, they cannot be assumed to improve the long run fiscal situation.
“New immigrants, particularly if they come on work visas, may generate a favourable balance of extra tax revenue relative to extra public spending for some years.
“But immigrants who stay grow older and have children, so the favourable tax to spending balance does not persist.
“And even when the favourable fiscal effects persist, they may do so largely because government spending on public services [particularly on health and education] falls in per capita terms and the quality of those services is eroded as population rises.”
There is growing public concern about the impact of rising immigration. A survey last month commissioned by the Onward think tank showed almost nine in 10 UK parliamentary constituencies want to see levels reduced and controls tightened.
However, European human rights chiefs have criticised UK ministers for using “inflammatory” and “derogatory” language against migrants and demanded Rishi Sunak rethink his stop the boats legislation.
Sparking a fresh clash with the UK, the experts from the Council of Europe warned that his plans to deport illegal migrants to Rwanda eroded “basic safeguards that protect persons from being potentially subjected to torture and inhuman or degrading treatment or punishment.”
They urged the Prime Minister to reverse his proposals and ensure that all foreign nationals arriving in Britain were able to apply for asylum here before any attempt to remove them to their home country or a safe third nation such as Rwanda.
The committee’s criticism follows a fact-finding visit last year to the UK by the Council’s committee for the prevention of torture.
It said: “Harsh immigration detention policies do not deter persons in need from making dangerous journeys to reach what they perceive as a safe country.
“Persons seeking refuge need to be provided with a clear route by which they can find safety for themselves and their families, preferably close to their country of origin but if that is not possible further afield, including Europe.”
The OBR expects Britain’s welfare bill to top £300bn this year as payments rise in line with soaring prices.
Much of the increase in the latter half of the decade is thought likely to be driven by spending on “health and disability benefits, reflecting rising caseloads” and spending on pensioners.
The OBR also projects that spending on health and disability benefits for people of working age will rise from £60bn this year to just over £77bn in 2028.