"What we've got to do is to make sure that everybody understands that the cost of miscalculation on the borders of both Ukraine and of Poland would be enormous," Johnson told a panel of senior UK lawmakers.

"I think it would be a tragic, tragic mistake for the Kremlin to think there was anything to be gained by military adventurism."

The British leader's comments, during a wide-ranging question-and-answer session with a watchdog committee of MPs, came as the West grows increasingly concerned about Russian troop movements at the Ukrainian border.

Moscow has also been accused of helping to orchestrate a crisis that has left hundreds of migrants from the Middle East trapped on the Belarus-Poland border.

Thousands of troops have been deployed on both sides, and after Belarus recently held joint drills with Russian paratroopers.