Britain is prepared to deploy troops to protect Nato allies in Europe if Russia invades Ukraine, Boris Johnson has said in a Commons statement, the BBC reports.

The prime minister warned that President Putin would face "ferocious" Ukrainian resistance and "many Russian mothers' sons will not be coming home".

The UK and its allies would also respond swiftly and "in unison" with "severe" economic sanctions, he added.

Russia, which denies plans to invade, has built up troops at the border. Some 100,000 Russian soldiers have been deployed.

The US has put 8,500 troops on alert to deploy at short notice, which Russia said caused it "great concern".

Members of the Nato alliance, including Denmark, Spain, Bulgaria and the Netherlands, are sending more fighter jets and warships to Eastern Europe to bolster defences in the region.

In a statement made just hours after the announcement of a police probe into No 10 lockdown parties, Mr Johnson said: "The British Army leads the Nato battle group in Estonia and, if Russia invades Ukraine, we would look to contribute to any new Nato deployments to protect our allies in Europe."