Previously, ministers had intended to delay scrapping free movement until new rules are in place, with a bill stuck in the Commons and fierce rows over what those rules should be.
The Liberal Democrats condemned the acceleration as “brutal”, warning it exposed Ms Patel as being “completely detached from reality”.
And the organisation representing more than 3 million EU citizens in the UK said: “This will open the door to discrimination. There are no systems in place.”
The dramatic shift comes despite the government declining to bring forward the stalled bill which would end free movement under a slower timetable, for fear of a Commons ambush.
Instead, Ms Patel believes she can act through secondary legislation, in a way that would bypass MPs of all parties who would oppose it.
Home Office officials have been sent to Singapore to copy its solution to technical issues, with the home secretary convinced it can be introduced quickly.
But Ed Davey, the Lib Dem home affairs spokesperson, said: “It is completely detached from reality and is next chapter in the never-ending saga of the utter mess they are making of Brexit.
“What would this mean for EU citizens who have made their home in the UK who have travelled abroad when they try to return?
“Are the government seriously suggesting an NHS nurse who is an EU national may not be allowed to return to the country if they happen to have been on holiday? It is absurd.”
Sir Ed also disputed that Ms Patel could avoid passing an act of parliament, describing any such attempt as “outrageous”.
And Nicolas Hatton, head of the3million group of EU citizens in this country, said: “There are no systems in place and nothing is ready. This is a political gesture, but it will have a real impact on people’s lives.
“This will open the door to discrimination. How will they distinguish between the ‘legacy people’, those already here, and those who will arrive afterwards?”
Sajid Javid, Ms Patel’s predecessor, had dismissed a day-one end to free movement as not “practical” for employers and others, saying: “There will need to be some kind of sensible transition period.”
The new plan may be viewed as part of efforts to force the EU into reopening Brexit negotiations, by signalling an uncompromising stance that would also cause huge upheaval across the Channel.
The government will not bring back the existing immigration bill because it fears it will be hijacked by MPs seeking to block a no-deal Brexit, who could table amendments.
In any case, business and public service leaders, as well as some ministers, are fighting a mooted £30,000 salary threshold for would-be immigrants – fearing severe staff shortages.
Boris Johnson further muddied the waters when he said advisers would now be told to work up plans for “an Australian-style points based system”, declining to set any limit on numbers.
In the Commons last month, the prime minister made no mention of the bill, instead telling MPs: “No one believes more strongly than me in the benefits of migration to our country.”
A senior Home Office source told The Independent that Ms Patel wants free movement “to end on 31 October”.
“Priti wants to toughen the Home Office’s stance,” the source added. “She thinks Saj [Mr Javid] did a great job but, with a new prime minister and new priorities, changes needed to be made.
“For a start that means properly preparing for no deal, it’s clear those in the centre had no intention of preparing for no deal.”
No 10 declined to comment on the new approach, but it is believed to be endorsed by Downing Street and Dominic Cummings, the controversial chief aide.
Last autumn, Mr Javid said of ending free movement immediately: “We’ve just got to be practical.
“If there was a no deal, we won’t be able to immediately distinguish between those Europeans that were already here before 29 March [the then-exit date], and those who came after. There will need to be some kind of sensible transition period.”
A Home Office spokesperson said: “The home secretary has been clear in her intention to take back control of our borders and end free movement after 31 October.
“Ending free movement means we are no longer required to give unlimited and uncontrolled access to those from EU countries when they are coming here seeking to work.”
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