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BERLIN (Reuters) - The head of German Chancellor Angela Merkel’s party warned European politicians that they risk undermining pro-European Union voices in Britain if they adopt too harsh a tone toward the country as it negotiates to leave the bloc.

Last week’s Salzburg summit, at which Theresa May was told her so-called “Chequers Plan” for a future relationship with the EU would not pass muster, was widely presented in the British press as a debacle for the country’s Prime Minister.

But Annegret Kramp-Karrenbauer, general secretary of Merkel’s Christian Democrats, said EU politicians must exercise care in their choice of tone for fear of weakening proponents of continued British membership or those who want the future Britain-EU relationship to be as close as possible.

EU leaders were unanimous at the Salzburg summit in saying that May’s plan for partial membership of the EU’s customs union for goods amounted to the “cherry picking” which the bloc had ruled out for Britain after it leaves.

But some comments by European leaders were widely seen by British newspapers as insulting, especially a social media post by European Council President Donald Tusk of him and May captioned with him offering her cake with “no cherries”.

Kramp-Karrenbauer said the matter had been raised at a meeting of the party leadership.

“It very much rides on the tone used,” she said, adding that there was a “very fine line” between making clear to Britain the limits of what it could ask for and alienating potential allies with excessive stridency.