The UK should start mobilising and beef up its military now, a new bombshell report has warned. Military planners are being told to urgently expand recruitment into the armed forces, with warnings that the “window of orderly action is limited".

The report, from RUSI think-tank, is expected to raise concerns in Whitehall, amid growing tensions over UK defence spending, after the Prime Minister last week failed to confirm when the government’s military funding plan would be released. The analysis suggests Britain’s land forces may need to double in size to address the growing threat of conflict with Russia. Paul O’Neill, Senior Associate Fellow at RUSI, said Britain should have increased its military power "10 years ago”.

He told The Sun that Britain has the "tradition" of "doing these things too late". O’Neill said: "We need to grow the capacity to absorb more reserves, then bring more in."

Tensions between Russia and the UK have intensified in recent weeks, with Russia warning that it is preparing appropriate measures in response to attempts by UK authorities to seize vessels linked to the so-called shadow fleet.

Andrei Kelin, the Russian ambassador to Britain, told Britain they should "think carefully" about the consequences, including legal ones, of actions taken against Russian-linked vessels.