LONDON, Jan 24 (Reuters) - Britain supports Venezuela's opposition leader Juan Guaido as head of the democratically elected National Assembly, a spokesman for Prime Minister Theresa May said on Thursday, adding that a 2018 presidential poll was neither free nor fair.

Guaido declared himself interim president on Wednesday, winning backing from Washington and parts of Latin America and prompting socialist Nicolas Maduro, Venezuela's leader since 2013, to break relations with the United States.

"The 2018 presidential election in Venezuela was neither free nor fair, so the regime's basis for power is deeply flawed. We fully support the democratically elected National Assembly with Juan Guaido as its president," May's spokesman said.

"In relation to the U.S, we think it is totally unacceptable for Venezuela to cut off diplomatic ties. The solution to this crisis lies in working to find a peaceful and diplomatic solution, not in expulsions." (Reporting by William James, writing by Elizabeth Piper; editing by Stephen Addison)

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