EU migration to the UK over the last decade has been underestimated, statisticians have admitted, as the reliability of official data was formally downgraded.

In a humiliating admission by the Office for National Statistics (ONS), its migration statistics quarterly report – which the UK government use to inform policy – has been reclassified as “experimental”.

The move comes after statisticians discovered earlier estimates of net migration between 2009 and 2016 from the EU8 countries - Czech Republic, Estonia, Hungary, Latvia, Lithuania, Poland, Slovakia, Slovenia – were too low.

The number of people from outside the EU moving to the UK was overestimated, meaning the headline net migration figure – the difference between those leaving and arriving – was broadly unchanged from unadjusted estimates.

As prime ministers, David Cameron and Theresa May stuck to their political target to reduce net migration to the tens of thousands – a target both failed to meet 37 quarters in a row. Boris Johnson ditched that target on entering No 10.

The admission that tens of thousands more EU immigrants have moved to Britain comes at a time of intense debate over freedom of movement, as the home secretary, Priti Patel, plans to bring it to an end in the event of a no-deal Brexit.

The scale of the undercount is not yet known. However, the ONS said the estimate of EU net migration for the year to the end of March had been increased by 16% (29,000). This means an increase in the estimated EU migration from 178,000 to 207,000 for the year ending March 2016.

The adjustments do not increase the estimate of the EU citizen population in the UK, which comes from a different data source and remains at 3,640,000 for 2018.

Madeleine Sumption, the director of the Migration Observatory at the University of Oxford, said: “We have been pointing out for a while that something wasn’t quite right in the net migration statistics, and that the comparison of EU v non-EU net migration did not seem plausible.

“This matters because for the past nine years the UK policy debate has been fixated on a single data source, which couldn’t bear the load that it was forced to carry.

“Whether the question is how to meet the net migration target or what to do about international students, the truth is that the data were simply not robust enough to be picked apart in such detail. The quarterly drumbeat of migration statistics that has become a feature of the UK migration debate arguably just overdramatised small changes in figures that were always quite uncertain.”

The migration statistics quarterly report relied heavily on the international passenger survey (IPS), which collects information about passengers entering and leaving the UK. The IPS conducts between 700,000 and 800,000 interviews a year, of which more than 250,000 are used to produce estimates of overseas travel and tourism.

The ONS said its research suggested migrants’ uncertain intentions on their likely length of stay in the UK had impacted the way estimates were calculated.

Iain Bell, the deputy national statistician, said: “Our findings so far support previously published estimates of headline net migration. However, while the headline trend is broadly unchanged, our programme of work has revealed some differences between data sources.

“As a result, EU migration may have been somewhat higher and non-EU migration somewhat lower than previously published. These differences largely reflect uncertainty in intentions among specific groups.”