British Prime Minister Boris Johnson has agreed with other world leaders to present a “united front” against Russian threats towards Ukraine, and has vowed to use “all the economic and diplomatic tools” to prevent Russian aggression. Johnson spoke about the situation in Ukraine to U.S. President Joe Biden, Italian Prime Minister Mario Draghi, French President Emmanuel Macron, and German Chancellor Angela Merkel on Monday. According to a Downing Street spokesman, the leaders “emphasised the need to provide a united front in the face of Russian threats and hostility.” They “called on Russia to deescalate tensions and reaffirmed their staunch support for Ukraine’s territorial integrity,” the spokesman said. “The prime minister said the UK would continue to use all the economic and diplomatic tools at its disposal to prevent any Russian aggression against Ukraine,” he said. Biden and Putin are due to speak in a video call on Tuesday amid rising …