Hundreds of British troops are to be deployed to eastern Europe as part of a show of strength by Nato in the face of an increasingly assertive Russia.
At the Nato summit in Warsaw, PM David Cameron will announce the deployment of a 500-strong battalion to Estonia with a further company of 150 troops to be stationed in Poland "on an enduring basis".
Britain is also to take over the leadership of the Nato Very High Readiness Joint Task (VJTF) from next year with 3,000 troops in the UK and Germany on standby to move with as little as five days notice.
The move comes amid continuing concerns among the Western alliance regarding the intentions of President Vladimir Putin following Russia's annexation of Crimea from Ukraine in 2014.
But it will also be seen as a signal of Mr Cameron's determination that Britain should continue to play a leading role on the world stage in the wake of last month's referendum vote to leave the EU.
"This will be a summit where you will see Britain assert itself as one of the most crucial elements in the Nato alliance," one official said.
"Not only are we going to be steadfast in our support of Nato, but we are prepared to back that up with boots on the ground."
Speaking ahead of the meeting, Mr Cameron said: "This summit is a chance for us to reiterate our strong support for Ukraine and our other Eastern allies to deter Russian aggression.
"Actions speak louder than words and the UK is proud to be taking the lead role, deploying troops across Eastern Europe. It is yet another example of the UK leading in Nato."
The Prime Minister will also use his attendance at what will be his final Nato summit to underline the importance of Britain's continuing commitment to meet the alliance target of spending 2 per cent of GDP on defence, as it seeks to persuade more member states to match the commitment.
His intervention is likely to be seen as being aimed as much at whoever wins the Conservative leadership race to succeed him in No 10 when he stands down in September as it is his fellow Nato leaders.
"There can be no backsliding on this issue," a government source said.
"The PM is very clear that the 2 per cent commitment is absolutely crucial to Nato going forward."
