"Venezuela stands on the brink of disaster and Nicolas Maduro's government must stop before it is too late.
"The country is turning on itself -- more than 100 have died already -- and democracy and basic rights are in jeopardy," Johnson said in a statement yesterday.
Britain's reaction came a day after Venezuela's election of a new "Constituent Assembly" superseding the country's legislative body, the opposition-controlled National Assembly.
The move hands almost total power to Maduro's Socialist party and was described as "dubious" by Johnson.
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Britain's foreign minister said the election has "dramatically deepened the problems and ramped up tensions".
"It is time for the government to see sense and start working with the opposition on a way forward that brings the people of Venezuela back together," he added.
Sunday's vote has also been criticised by the European Union and countries including the United States, which on Monday imposed sanctions on Maduro and branded him a "dictator".
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