The most important question in the wake of today’s immigration data is this – how do you lose more than 300,000 people?

The Office for National Statistics (ONS) dropped a bombshell into the already heated debate about migration, as it admitted its earlier figures were way, way out.

In sweeping revisions to figures it published just six months ago, the agency said that over the last three years net migration was 307,000 higher than previously thought.

Net migration across 2021, 2022 and last year was, in fact, more than 2.2million rather than the 1.9million its estimates suggested in May.

Within those three years, net migration peaked at 906,000 in the 12-month period to June last year, which was 166,000 higher than previously thought, the ONS admitted.

An overlapping period – the 2023 calendar year – saw an even higher revision of 181,000.

To be clear on the terminology, net migration is the difference between the number of migrants arriving in Britain and those emigrating.

And today the ONS has confirmed its previous data on both components of that sum were wrong.

Immigration levels have been higher than previously thought – 1.3million last year, rather than 1.2million – and levels of emigration were over-estimated.

In 2023, 450,000 emigrated rather than the ONS’s previous estimate of 532,000 – so more people are staying here.

The nub of the error is this.

ONS statisticians had been assuming that large numbers of foreign nationals whose original UK visas had expired and who had obtained a new visa were leaving the country.

This assumption was incorrect or, at least, was applied too broadly.

It turns out that many of those foreign nationals were remaining in Britain.

The ONS now has access to additional data from the Home Office which digs deeper into the numbers who stay, leading the statistics body to unveil its major revisions to migration data today.

Its adjustments ‘did not go far enough in reducing emigration estimates’ and ‘failed to fully adjust for these populations’, ONS documents say.

‘This meant these individuals were still being incorrectly classified as long-term emigrants.’

The ONS privately admitted to me today that its previous assumptions were ‘too generous’.

The ONS director Mary Gregory insisted today that net migration is ‘beginning to fall’, as the latest figure shows a dip from the 906,000 peak to 728,000 in the 12 months to June this year.

But the sheer scale of the ONS’s recalculations has done nothing to reassure its critics.

One Conservative backbencher who has long held concerns about the way Britain gathers immigration data, Neil O’Brien, told me there needs to be a ‘total overhaul’ in the wake of today’s ‘incredible’ figures.

ONS officials suggested that the kind of seismic increases in their data which we have seen today will not be repeated in future sets of data.

The new sources of data are bedding in, they assured me, and aspects of their data-gathering that led to the huge jumps are already being taken into account in their most recent calculations.

The trouble is, the ONS have given me similar assurance in the past. It is far from certain that we won’t see large revisions in these ‘falling’ net migration figures, too.

Successive government over the last 20 years have been pledging to improve Britain’s border control systems so that everyone is ‘counted in and out’.

This, of course, is the only way to ensure the Home Office knows how many foreign nationals are here, and particularly how many people have overstayed their visas.

But the technology is still not in place, after decades of talk.

A new electronic travel authorisation system is being introduced, which ministers have previously said will allow a full tally to be compiled as part of a ‘fully digital border’.

Whether it works or not will be another matter.

The government’s long-standing failure to produce reliable figures grows more intolerable by the day, as immigration flows grow.

Legitimate concerns about pressures on housing, the NHS, benefits, education and other public services cannot be properly measured without the full facts.

It really is time, as Mr O’Brien suggested to me, for a complete ‘re-boot’ of the stats.

Only then can there be a proper, informed debate and ministers can draw up policies that will respond to the real situation on Britain’s streets.