The European Parliament has criticized the U.K.’s proposals on EU citizens’ rights as a “damp squib” and has vowed to reject a Brexit deal if Britain does not offer more.

Chief negotiator Guy Verhofstadt and leaders of four of the parliament’s main groups wrote in a joint letter to newspapers that Britain’s plans for the three million EU citizens expecting to remain in the U.K. post-Brexit “fall short” of what they are entitled to and what U.K. nationals are being offered in the EU.

“The proposal falls short of its own ambitions to “put citizens first”. If implemented, it would cast a dark cloud of vagueness and uncertainty over the lives of millions of Europeans,” the representatives wrote.

In outlining plans for citizens’ rights, the EU had proposed that Britons and Europeans would keep the same rights and protections they currently receive under European law. However, the British proposal, delivered three weeks later, suggests that Europeans’ rights within the U.K. would be significantly reduced from the day after Brexit, including curtailing voting rights and income thresholds.

“The British proposal carries a real risk of creating a second class of citizenship,” the group of eight signatories wrote, claiming that the plans backtrack on Brexiteers’ pledge that EU citizens would be treated “no less favourably than at present.”

The leaders of the four groups, who account for two-thirds of the votes in European Parliament, added that they would veto any deal which places existing EU citizens’ rights in jeopardy.

“The European Union has a common mission to extend, enhance and expand rights, not to reduce them. We will never endorse the retroactive removal of acquired rights.

“The European Parliament will reserve its right to reject any agreement that treats EU citizens, regardless of their nationality, less favorably than they are at present. For us, this is a question of basic fundamental rights and values, which are at the heart of the European project.”

Brexit negotiations kicked off last month shortly after British Prime Minister Theresa May failed to increase her parliamentary majority in a snap general election.

British and EU leaders now have just over a year and a half to negotiate and agree a deal, which will then go to a vote before European Parliaments.

The deadline for a striking a deal is March 2019. The European Parliament has said that an extension beyond this would be “unthinkable.”

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