Eighty British MPs wrote to Tusk saying that Britons living in European Union countries must have their rights protected after Britain leaves the bloc following its June referendum vote.
"Just like you, I would like to avoid a situation where citizens become 'bargaining chips' in the negotiation process," Tusk wrote in reply to the British parliamentarians, who had first used the phrase.
"For this not to happen, we will need precise and comprehensive solutions, which, other than nice-sounding expressions, will provide citizens with genuine guarantees of security," the former Polish premier wrote.
Prime Minister Theresa May has said she will initiate the two-year exit proceedings in March.
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Tusk said that the EU was ready to start negotiations whenever London was "but that can only happen on the condition that Article 50 has been triggered", and that it remained a decision for Britain.
Tusk also lashed out at the British MPs for saying that the European Commission's Brexit negotiator, Frenchman Michel Barnier, had caused "anxiety and uncertainty" by refusing to negotiate with London.
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