The former Liberal Democrat leader will argue that remaining in the bloc is essential for tackling a mounting global security threat.
And he will warn the UK will be reduced to a “cork bobbing around behind other people’s ocean liners” if voters back Brexit.
Lord Ashdown, a former Royal Marine who was High Representative for Bosnia and Herzegovina from 2002 to 2007, will insist he is “no believer in a European army” – which Leave groups say would be a consequence of continued membership.
“But I do believe that the nations of the EU have to get more serious about hard defence in a troubled age,” he will say. In a speech in London today he will say: “I know from my time in Bosnia that the EU is the world’s greatest soft power peace builder – and God knows, against the rising tide of global conflict that is needed now more than ever. But soft power is not enough.”