“If I wanted, in two days I could have Russian troops not only in Kiev, but also in Riga, Vilnius, Tallinn, Warsaw and Bucharest,” Mr Putin allegedly told President Petro Poroshenko of Ukraine, reported Süddeustche Zeitung, a German newspaper.

If true, this would be the first time that Mr Putin has threatened to invade Nato or EU members. Any threat to send Russian troops into the capitals of Latvia, Lithuania, Estonia, Poland and Romania would cause grave alarm among Western leaders.

If Mr Putin were to act on this, Britain could find itself at war with Russia. All five countries mentioned in this alleged conversation are members of both the EU and Nato. They are covered by the security guarantee in Article V of Nato’s founding treaty, which states that “an attack on one is an attack on all”. In a speech in Tallinn earlier this month, President Barack Obama confirmed Nato’s commitment to this doctrine.