Theresa May's offer on residency rights for EU citizens following Brexit is "a first step, but not sufficient", the president of the European Commission has said.
Jean-Claude Juncker's comment came shortly after the Prime Minister accepted that her proposals, which envisage a new "settled status" for EU nationals, will be contested by Brussels, with battle lines likely to be drawn over jurisdiction and the cut-off date for eligibility.
Meanwhile the UK offer was dismissed as "pathetic" by a group campaigning for an estimated three million European expats living in the UK.
Under the proposals which Mrs May outlined to fellow leaders at a Brussels summit on Thursday, settled status will be available to all EU nationals who have been in the UK for five years, granting them the same rights as British citizens to healthcare, education, welfare benefits and pensions.
Those with a shorter period of residency will be able to stay on to reach the five-year threshold and those arriving after a yet-to-be-defined cut-off date will have a "grace period" to regularise their status.
Arriving for the second day of the summit on the anniversary of the referendum on June 23 last year, which paved the way for Brexit, Mrs May said EU expats should take "reassurance and confidence" from the package.
"I want to reassure all those EU citizens who are in the UK, who have made their lives and homes in the UK, that no one will have to leave. We won't be seeing families split apart," she said.
"This is a fair and serious offer. I want to give those EU citizens in the UK certainty about the future of their lives, but I also want to see that certainty given to citizens who are living in the EU."
But the co-chairman of the 3Million movement, Nicolas Hatton, retorted: "There is something slightly pathetic about the Prime Minister's proposal which makes no reference to the detailed, comprehensive offer tabled by the EU. The Prime Minister described her proposal as fair and serious. It's neither fair nor serious."
The 3Million said Mrs May's proposals failed to end uncertainty over the reunification of families, the right to work, the recognition of professional qualifications and the ability to retain UK rights when moving between and working across different European countries.
The group said the UK offer lacked the lifetime guarantee of rights and enforcement by the European Court of Justice included in the formal proposal already tabled by the EU.
Mrs May has promised further details in a Government paper to be published on Monday. She also promised the system will be streamlined, doing away with the 85-page permanent residency application form which has provoked loud complaints.
But the cut-off date for residency rights - falling somewhere between the March 29 2017 date of Britain's formal notification of intent to leave and the date of Brexit, expected on March 29 2019 - will not be known until later in the negotiation process.
It is thought the UK is reserving the option of setting an early cut-off for residency rights in case there is a late surge of migrants arriving as Brexit approaches.
But the introduction of a grace period raises the prospect that large numbers arriving during withdrawal negotiations may be allowed to remain.
And the prospect of an early cut-off conflicts with the EU proposals, which would grant residency rights all the way up to the final date of withdrawal.
Mrs May set her face against European Court of Justice involvement as she briefed her fellow leaders over dinner on Thursday, telling them: "The commitment that we make to EU citizens will be enshrined in UK law and will be enforced through our highly respected courts."
She acknowledged that elements of the package will be challenged by Brussels, saying: "Of course, there will be details of this arrangement which will be part of the negotiation process."
German Chancellor Angela Merkel said the proposals represented "a good start" but cautioned there were "many, many other issues" before Britain could reach agreement on a withdrawal deal.
Dutch Prime Minister Mark Rutte said there were "thousands of questions to ask" about Mrs May's proposals, and Austrian Chancellor Christian Kern described them as "a first step" which did not cover the situations of many EU citizens in the UK.
Belgian Prime Minister Charles Michel called it a "particularly vague proposal". Using a Flemish phrase to describe a dubious gift, he said: "We don't want a cat in the bag. We want the rights of EU citizens to be permanently guaranteed."
Labour's Sir Keir Starmer said the PM's plan was "too little too late", while London Mayor Sadiq Khan said it "does not come close to fully guaranteeing the rights of EU nationals living in the UK".
Liberal Democrat leader Tim Farron said the proposals "leave millions of people still facing unanswered questions over their futures here".
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