UPDATE 1-New UK finance minister Zahawi says PM Johnson must go

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LONDON, July 7 (Reuters) - Britain's new finance minister Nadhim Zahawi told Boris Johnson to resign on Thursday, less than 48 hours after the prime minister promoted him to the job, saying the crisis engulfing the government would only get worse.

"This is not sustainable and it will only get worse, for you, for the Conservative Party and most importantly of all the country," Zahawi said on Twitter after more than 50 ministers and aides resigned from the government.

"You must do the right thing and go now."

Zahawi did not say he had resigned, but said he went to the prime minister's office in Downing Street with cabinet colleagues on Wednesday evening to tell Johnson to "leave with dignity".

"I am heartbroken that he hasn't listened and that he is now undermining the incredible achievements of this government," Zahawi said in a letter.

"The country deserves a government that is not only stable, but which acts with integrity. Prime Minister you know in your heart what the right thing to do is, and go now." (Reporting by Kate Holton and Alistair Smout; Editing by William James and Michael Holden)