The UK is “flying blind” about the impact of new immigration policies because coronavirus restrictions mean that the country’s methods of quantifying net migration are suspended or working less well than normal, a leading expert on the issue has said.

Madeleine Sumption, director of the Migration Observatory at Oxford university, said there was “absolutely massive uncertainty” about what was happening to migration in and out of the UK because of the suspension of the international passenger survey, normally the main means of gauging migrant flow.

Data collection was suspended in March last year because of the difficulty in conducting face-to-face interviews at border points during the pandemic.

Figures for new national insurance numbers, usually another important means of monitoring migration, have also been disrupted by delays in issuance during the pandemic.

Ms Sumption was speaking as the observatory published a commentary studying how many migrants had left the UK during the pandemic. One counting method — the labour force survey — has suggested that the UK’s foreign-born population shrank by 840,000 to 8.3m between the third quarter of 2019 and the third quarter of 2020.

The commentary, Where did all the migrants go?, said there was “enormous uncertainty” about the labour force survey estimates and “compelling reasons” to believe they were not accurate. The survey assumed the UK’s overall population had kept growing as before during the pandemic, the commentary said. As a result, it had probably overestimated the decline in the country’s foreign-born population.

Nevertheless, the report conceded there had clearly been a fall in the UK’s population during the pandemic, especially in London.

The uncertainty comes as the UK rolls out a new immigration system following the end of free movement between the UK and the countries of the EU, Switzerland, Norway, Iceland and Liechtenstein. The new system, in place since January 1, makes it far harder to recruit workers from European countries but makes it far easier to recruit workers from elsewhere in the world.

There was “massive uncertainty” about what was going on with migration because all the normal data sources had been “hugely disrupted”, Ms Sumption said.

“This has left us flying blind just as the UK is introducing a new immigration system, and will make it more difficult to understand the impacts of new policies,” she said.

Last month, the Economic Statistics Centre of Excellence, an agency set up by the Office for National Statistics, calculated that up to 1.3m people born abroad had left the UK between the third quarter of 2019 and the same quarter of 2020.

Information about the size of the UK’s population is important for setting policy relating to the labour market and immigration and for gauging the success of initiatives such as the EU Settlement Scheme, which aims to register the UK’s European-born residents.

Jonathan Portes, senior fellow at the UK in a Changing Europe think-tank at King’s College London, said the Migration Observatory’s research confirmed his view that London’s population had fallen sharply during the crisis, thanks to an outflow of foreign-born workers.

But he added: “The wide variance in estimates of just how many people have left the country shows just how many questions remain unanswered.”

The Home Office insisted it retained a “clear, accurate” picture of migration into the UK through its immigration statistics.

“The Home Office continues to work closely with the ONS on challenges specifically related to migration, such as the withdrawal of the international passenger survey, and to transform statistics and improve their quality in the longer term.”