The plot was foiled only hours before it was due to be carried out, but would have caused heavy bloodshed and plunged the tiny country into turmoil on the eve of becoming Nato’s 29th member.

The allegation came as Sergei Lavrov, Russia’s foreign minister, criticised Nato as a “Cold War institution” whose expansion had led to unprecedented tensions in Europe over the past 30 years.

The planned Montenegro coup, scheduled for Oct 16 last year, was one of the most blatant recent examples of an increasingly aggressive campaign of interference in Western affairs, Whitehall sources told the Telegraph.

Boris Johnson, the Foreign Secretary, and his US counterpart, Rex Tillerson, are understood to have discussed the issue last week at their first face-to-face meeting.

Interpol is now hunting two Russians the Montenegrin government says are intelligence officers who hatched the plot.

The pair spent months overseeing the recruitment and equipping of a small force of Serbian nationalists to attack the parliament building, disguised as local police, and kill Milo Djukanovic, the prime minister at the time.