The main body of British troops are on their way to Estonia as part of a major NATO mission in the Baltic states to deter Russian aggression.
Around 200 soldiers, from 5th Battalion, The Rifles left RAF Brize Norton. They will join 120 service personnel who flew to the country a few weeks ago.
The UK is taking a leading role in NATO's "enhanced forward presence", aimed at reinforcing the Alliance's eastern border.
The 1100-strong battlegroup will contain 800 UK troops.
Defence Secretary, Sir Michael Fallon says this deployment is about the UK showing solidarity with its NATO allies
The UK-led Estonia battlegroup is one of four NATO multinational deployments to eastern Europe. Other NATO allies are sending forces to Latvia, Lithuania and Poland.
UK troops head to Estonia
Lt Col Mark Wilson of 5 RIFLES said the British force would "integrate into the Estonian defence force, and bolster their capabilities".