More than 100 UK troops are to fly out to Estonia to take part in a military exercise, as tensions between Ukraine and Russia continue.
The troops are to be joined by Defence Secretary Philip Hammond. The military exercise will mark the first time UK military forces have been sent to Eastern Europe since the unrest in Ukraine began.
BBC defence correspondent Jonathan Beale said the move was "an act of reassurance" to the UK's Baltic allies. He added: "But it is also meant as a signal to Russia that Britain is ready to play its part in defending the alliance."
The troops, reported as being from the 1st Battalion The Duke of Lancaster's Regiment, will leave from RAF Brize Norton in Oxfordshire on Friday.
Last month four RAF Typhoon fighter jets left their base in Lincolnshire and flew to nearby Lithuania to help carry out Nato's air policing role of the Baltic states.
As part of his trip, Mr Hammond is also expected to visit the crew of those fighter jets. The US announced in April it had also sent troops for military exercises, but to Poland, which borders Ukraine. An initial 150 US soldiers were followed by a further 450 within days.
At the last estimate some 40,000 Russian troops are stationed near the Ukrainian border, but Moscow is reported to have expressed concern about Nato's increased activity in the area. Eastern Ukraine has a large Russian-speaking population and was a stronghold for Ukraine's former president Viktor Yanukovych before he was overthrown by protesters in February.
Russia then annexed the Crimean peninsula - part of Ukraine but with a Russian-speaking majority - in a move that provoked international outrage.The crisis has since plunged East-West relations to their lowest point since the Cold War, defence experts say.