Britain is considering making a major NATO deployment as part of a plan to strengthen Europe's borders in response to Russia massing troops on the border with Ukraine, the government said on Saturday.
Britain has said that any Russian incursion into Ukraine would be met with swift sanctions and would be devastating for both sides.
Prime Minister Boris Johnson is due to visit the region next week, and also will speak to Vladimir Putin by phone.
Johnson is considering the biggest possible offer to members of the NATO defence pact in the Nordics and Baltics, which would double troop numbers and send defensive weapons to Estonia, his office said.
"This package would send a clear message to the Kremlin – we will not tolerate their destabilising activity, and we will always stand with our NATO allies in the face of Russian hostility," Johnson said in a statement.
"I have ordered our Armed Forces to prepare to deploy across Europe next week, ensuring we are able to support our NATO allies."
Officials will finalise the details of the offer in Brussels next week, with ministers discussing the military options on Monday.
Stepping up diplomatic efforts after facing criticism for not doing enough, Johnson will make a second trip to meet NATO counterparts early next month, his office said.
Britain's foreign and defence ministers will also both go to Moscow for talks with their Russian counterparts in coming days, with the aim of improving relations and de-escalating tensions.