Over 1m EU nationals could move to the to Britain in the next two years, warned top Breixteer
It puts fresh pressure on the Prime Minister not to wait until Britain actually leaves the European Union in 2019 to curb free movement of people which lets all EU citizens work and live in each other's countries.
Richard Tice, co-chairman of the Leave Means Leave clean Brexit campaign, said his calculations, based on recent years' National Insurance Number (Nino) registrations by EU workers, should be a wake-up call to the Government.
He hailed Mrs May's hard-hitting speech last week making clear her intention to take Britain out of the EU single market and regain control over immigration.
But he was fearful about the impact of doing nothing to tackle uncontrolled EU migration while we remain members of the bloc, after Mrs May failed in her speech to answer Leave Means Leave's previous call to announce the end of free movement as far as Britain is concerned.
Mr Tice told the Daily Express: "Last week the Prime Minister set out a positive vision of a Britain outside of the EU, free from the single market and the restrictive elements of the customs union.
"She made it clear that Britain will take back control of its borders – a commitment we welcome – and we support the Prime Minister as she goes forward to deliver on these pledges.
We do, however, have serious concerns about EU immigration
Richard Tice, Leave Means Leave
"We do, however, have serious concerns about EU immigration in the period between now and when Britain formally leaves the EU.
"Since 24th June 2016 it has been clear that our uncontrolled EU migration policy was likely to change and the Prime Minister confirmed this last week. It is now appropriate to clarify a timeframe.
"EU citizens are now well aware that freedom of movement will no longer continue but until a cut-off point is set, the British borders are open to everyone in the other 27 member states.
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"If freedom of movement for EU citizens continues for the next two years until we leave the EU, we could easily see another one million to 1.25 million extra EU migrants move to Britain."
Mr Tice said that estimate is a conservative one, based on Nino registrations, but is more accurate than the International Passenger Survey which the Government uses for its official migration statistics.
"The IPS survey only samples 0.2 per cent of travellers and only includes people who volunteer that they will stay for more than one year," explained Mr Tice.
"The potential for error and abuse is enormous in the government numbers."
The statement has put pressure on Theresa May to curb free movement before leaving the EU
His own estimate did not, he added, take into account the extra pull factor of Britain's looming departure from the EU making this "the last chance saloon" for people to secure the better prospects offered in the UK than elsewhere in the bloc, nor count non-working family members who arrived with the workers.
Mr Tice concluded: "Our hospitals, schools, transport and housing stock cannot cope as it is.
"We urge the Prime Minister to announce that the EU freedom of movement changes will take effect from now, so that the promise to take back control of Britain's borders can be delivered.
"This will help to avoid the pressures on our public services becoming a national crisis."
Latest Office for National Statistics figures showed net migration – the number of people coming to the UK minus those leaving – was a near-record 335,000 in the year to June 2016 including 189,000 EU citizens.
The huge number of EU migrants could come to the UK unless Mrs May regains control of the borders
A record 284,000 EU nationals moved to the UK in that period.
Numbers of European nationals getting Nino numbers is much higher. In the year to September 2015, the ONS said some 257,000 EU migrants came to live here while 630,000 Nino numbers were allocated to EU nationals in the same period.
In the year to September 2016, there were 629,000 Ninos issued to EU workers and 195,000 to non-EU workers.
The ONS says the IPS survey is still the best way to measure long-term migration, because Nino registrations include people coming for less than a year.
Alp Mehmet, vice chairman of MigrationWatch UK, said his organisation prefers to use net migration figures, although the gap with Nino registrations was not wholly explained by the short stay issue.
"But certainly since the referendum on June 23 there is there is little evidence to suggest there is a slowing-down of European migration and there is no reason to believe it will ease any time soon," he said.
Richard Tice says he supports the Prime Minister as she moves to deliver her Brexit promises
"There is no sign that Britain is becoming less attractive as a result of the vote."
Although EU countries can invoke safeguard mechanisms in times of crisis it is not clear if Britain would be allowed to do so just to reduce numbers.
Mr Mehmet said it was not legally possible for Britain to opt out of EU free movement until we actually leave the bloc but border control must be the longterm aim: "Ultimately, what is absolutely essential longer term is that we do stop free movement, otherwise we will be continuing to add to our population in the way we have been projecting and the half of net migration which is accounted for by EU nationals will just continue uncontrolled."
Labour leader Jeremy Corbyn yesterday SUNDAY insisted to Sky News that his focus with regard to free movement was not reducing immigration numbers but tackling how employers "abused" the rules to import groups of workers and undercut local British wages.
EU nationals working in Britain including 55,000 in the NHS should be "respected and thanked" for their contribution, he insisted.
Mrs May has said she is eager quickly to guarantee the rights of EU citizens already living in Britain but only when other EU countries agree the same protections for British ex-pats after Brexit.