The Prime Minister announced a major overhaul of the immigration system as he accused the Conservatives of running “a one-nation experiment in open borders”.

On the day revised figures estimated net migration hit a record 906,000 in 2023, Sir Keir Starmer told a Downing Street press conference that his Government would publish plans “imminently”, in the form of a white paper, to bring down the number of people entering the UK.

But he also took aim at the previous government, saying the quadrupling of net migration since 2019 had been a deliberate policy of the Conservatives.

He said: “Failure on this scale isn’t just bad luck. It isn’t a global trend or taking your eye off the ball.

“No, this is a different order of failure. This happened by design, not accident.

“Policies were reformed deliberately to liberalise immigration. Brexit was used for that purpose, to turn Britain into a one-nation experiment in open borders.”

Thursday’s press conference followed the publication of figures from the Office for National Statistics (ONS) that showed the difference between the number of people arriving and leaving the country reached the higher than previously thought peak in the 12 months to June last year, after being revised up 166,000 from the initial estimate of 740,000.

The figures, covering the previous Conservative government’s administration before the general election, have since dropped by 20% and stood at 728,000 in the latest period for the year to June 2024.

A similar change has been made to the provisional figure for net migration in the year to December 2023, which was initially estimated to be 685,000 and is now thought to be 866,000 – an increase of 181,000.

Describing the numbers as “unprecedented”, Sir Keir said: “It is off the scale what has happened in four short years.”

Britain’s economy was now “hopelessly reliant on immigration” and the Government inherited an “utter mess” in the Home Office, he warned.

Separate figures published at the same time showed:

– The cost of the UK’s asylum system has risen to £5 billion, the highest level of Home Office spending on record and up by more than a third in a year.

– Some 35,651 asylum seekers were being housed in UK hotels at the end of September, up more than 6,000 since the end of June signalling the first quarterly rise for a year.

– More people were granted British citizenship in the first nine months of the 2024 than in any calendar year since 1962. The 268,481 grants were well above the previous 2013 record of 208,095.

The figures come after new Tory leader Kemi Badenoch admitted her party had failed on migration.

In a speech on Wednesday, Ms Badenoch said there had been a “collective failure of political leaders from all parties over decades” to grasp migration, adding: “On behalf of the Conservative Party, it is right that I as the new leader accept responsibility and say truthfully, we got this wrong.

“I more than understand the public anger on this issue. I share it.”

But Sir Keir said she had provided only a “chorus of excuses”.

In his press conference, he said the Government would reform the points-based immigration system to require companies employing foreign workers to also train British people, as well as crack down on abuse of the visa system.

He said: “Let me say directly to the people watching, where the last government failed you, this one will not. They drove immigration numbers up. We will get them down.”

But he refused to “pluck an arbitrary number” when asked to set a target, arguing this had “achieved absolutely nothing” when tried in the past, adding: “The way to get it down is the hard graft, not the gimmicks.”

Meanwhile Home Secretary Yvette Cooper announced a “landmark” deal with Iraq, intended to crack down on people smuggling and boost border security.

The ONS said although remaining high by “historic standards”, net migration is “beginning to fall”.

Former home secretaries James Cleverly and Suella Braverman, who have both courted hopes of becoming Tory leader in the past, claimed credit for the drop in numbers while they led the Home Office.

But shadow home secretary Chris Philp said net migration “remains far too high” and the UK needs stricter border controls.

Fellow Tory frontbencher Robert Jenrick, who quit as immigration minister in December 2023, said it was a “day of shame for the Conservative Party”, adding: “Our handling of immigration let the country down badly. The public are right to be furious.”

Reform UK leader Nigel Farage called the figures “horrendous”.

The revisions are a result of the ONS continuing to review its net migration figures as more complete data becomes available, as well as improving how it estimates the migration behaviour of people arriving in the UK from outside the European Union.

Better analysis of the number of people coming to the UK amid the conflict in Ukraine has also been taken into account.

Some 1.2 million people are estimated to have arrived in the UK in the year ending June 2024, while 479,000 are likely to have left.

This compares with 1.3 million who arrived in the UK in the year to June 2023 and 414,000 who left.

The drop in the overall level of net migration has been driven mainly by a fall in the number of dependants arriving in the UK on study visas from outside the EU.