Despite what he said was a "generous offer" from London, the European Parliament's chief Brexit diplomat Guy Verhofstadt said on Friday that Britain had not still promised to "fully guarantee the rights of EU citizens living in the UK" post-Brexit.

In a speech to other EU leaders in Brussels on Thursday evening, British Prime Minister Theresa May gave what UK officials said was "a clear commitment that no EU citizen currently in the UK lawfully will be asked to leave the country at the point that the UK leaves the EU."

May said EU citizens who have been resident in Britain for five years would be treated as equal to British nationals in terms of health care, education, welfare and state pension provisions.

She said those resident for less than five years would be allowed to build up rights to remain in the country.

Still not clear

But Verhofstadt said a lack of clarity remained about the cut-off date for those seeking to build up permanent residency in Britain, along with concerns about family reunification rights and uncertainty about jurisdiction.

Several other EU leaders also expressed skepticism about May's pledge.

European Commission President Jean-Claude Juncker described it as "a first step," before adding that "this step is not sufficient."

German Chancellor Angela Merkel said on Friday the proposals were "good start but not yet the breakthrough." After talks with May the night before, she said: "it became clear ... that there is a long road in front of us."

Germany's Foreign Minister Sigmar Gabriel was more outspoken, describing May's opening offer as "the minimum," adding that her pledge was "actually something that should be taken for granted."

Read more: Negotiation experts say positive result in Brexit talks 'nearly impossible'

Nasty surprise?

Belgium's Prime Minister Charles Michel was concerned the promise was "particularly vague," and warned it could contain a nasty "cat-in-the-bag-surprise."

May's comments were the first guarantee made by the UK government about residency rights since the British electorate voted by a narrow majority in June 2016 to leave the EU.

She called for reciprocity for Britons living in other EU states and for the two sides to agree on citizens' rights as soon as possible.

Read more: Britons want Brexit clarity one year on

Seeking to clarify any doubts, the British prime minister reaffirmed on Friday that "no one will have to leave. We won't be seeing families split apart."

May told reporters her government was making a "very fair and very serious offer" to her EU counterparts to guarantee the futures of around 3 million European citizens in Britain.

The issue of the rights of EU citizens living in Britain and more than 1.5 million Britons on the continent is a top priority in Brexit talks. The two-year formal Brexit negotiations began on Monday.

Departure from ECJ's reach

May, who is battling to retain her authority after losing her parliamentary majority in a snap vote in early June, has sought to offer an olive branch to Brussels after previously adopting a hardline stance towards Brexit negotiations.

But the Conservative Party leader has still not specified a cut-off date for those who will qualify for permanent residency once Brexit is finalized.

Furthermore, May rejected a demand for the European Court of Justice (ECJ) to oversee the process of guaranteeing EU residency rights and any resulting disputes. Earlier this month, the European Commission said the court should have "full jurisdiction."

The British leader is determined to remove Britain from the ECJ's reach, saying that "taking back control" of its laws was one of the reasons Britons voted for Brexit.

Meanwhile, May was due to publish a full report on this issue and present her proposal to the British parliament on Monday.

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