The UK is hosting a closed meeting on Thursday of senior military officers from the "coalition of the willing" as they draw up plans for a proposed peacekeeping force for Ukraine.
More than 20 countries are thought to be involved.
Prime Minister Sir Keir Starmer is expected to attend on Thursday afternoon after first visiting Barrow, where he is due to lay the keel of one of Britains next generation of nuclear-armed submarines.
Plans for a Western-led peacekeeping force for Ukraine are said to be moving to an operational phase.
Senior military officers from nations that make up the "coalition of the willing", led by Britain and France, are due to discuss how this would work in practice when they gather at the UKs Permanent Joint Headquarters in Northwood.
But there remain some major obstacles.
Russian President Vladimir Putin has said Moscow will not tolerate the presence of any Nato member troops in Ukraine, regardless of their role.
The US is also proving reluctant to provide the necessary air cover that prospective members say they need.
Sir Keir is expected to attend part of the meeting after first visiting Barrow where he will lay the keel of HMS Dreadnought, one of the next generation of ballistic nuclear submarines.
Earlier he visited the crew of one of Britains nuclear submarines, HMS Vanguard, as it returned to Scotland after patrolling the waters of the north Atlantic.
The prime minister told reporters the Kremlin respected the UKs nuclear arsenal because "weve got our own independent deterrent and were committed to Nato".
"What is obviously important is they appreciate that it is what it is which is a credible capability," he said. "And that it most certainly is."