There is a "convulsion of barbarity" that will destabilise the whole of the Middle East and could, "in due time" strike the West as well, Lord Ashdown told Good Morning Britain.
The former Liberal Democrat leader warned IS posed a "real case of wider threat", but tried to reassure voters airstrikes were legal and the UK had been invited to join the coalition by the Iraqi government.
An inquest has heard how a driver died when he lost control of his vehicle and careered into a car carrying Lord Ashdown and his wife before hitting a van.
Mark Hurford, 49, was pronounced dead at the scene of the crash, the A356 at Turnpike Cross, Somerset, on January 24 this year.
An inquest was read witness statements from Paddy and Jane Ashdown, who had been travelling to their home in Norton-sub-Hamdon.
Former Liberal Democrat leader Lord Ashdown said his wife had picked him up in her silver Renault Scenic from Crewkerne Railway Station at 4.50pm.
"One mile from home I heard my wife exclaim loudly," he said in a statement.
"At the same time, there was a massive crash and a jolt to the car.
East Somerset Coroner's Court heard Mr Hurford, of Crewkerne, was more than two-and-a-half times the drink drive limit as he drove home from work at 5pm.
Former Lib Dem leader Lord Ashdown warned the party to "stop it now" after a "plot of deep malice" to remove Nick Clegg was revealed.
Lord Ashdown told the BBC's Andrew Marr Show, "Anything you do now which is not getting out on the street, campaigning in the context of the next General Election ... is distraction and a dangerous distraction."
He said the Lib Dems have to "get out and start campaigning for the next election" following disappointing results at the local and European elections.
Former Liberal Democrat leader Lord Ashdown said he agrees with Nick Clegg after he said was "a matter of deep regret" that Lord Rennard has not apologised for his behaviour:
The technology used by Britain's intelligence agencies is "out of control", former Liberal Democrat leader Lord Ashdown has warned.
Lord Ashdown, a former special forces soldier and spy, called for an inquiry to address questions of privacy in the digital age.
He said surveillance should be targeted against individuals or groups, not against "the whole nation" as recent operations exposed by whistleblower Edward Snowden were.
But in an interview with The Guardian, which has revealed details of the activities of British eavesdropping agency GCHQ based on documents disclosed by US intelligence whistleblower Mr Snowden, Lord Ashdown defended the right of the state to intercept communications.
Former Liberal Democrat leader Lord Ashdown says he feels "depressed and ashamed" following last night's Syria vote.
He added he has seen children burning and the response from his country is "nothing to do with me."
Lord Paddy Ashdown has warned the House of Lords over making decisions over Syria based on lessons learned after Iraq.
Lord Ashdown said: "I remember well trying to persuade Parliament that we needed to intervene in Bosnia - when we were under shadow of Vietnam. Now here we are and I think living under the shadow of Iraq.
"But this is not Iraq, we are not putting boots on the ground and we are not invading and above all this is not George W Bush it's Obama."
Former Liberal Democrat leader Paddy Ashdown spoke to ITV News Presenter Alastair Stewart after Baroness Thatcher's funeral and said: "It's an opportunity to pay respects to a remarkable Prime Minister.
"You may agree with her or not one thing is without doubt; when she came to power Britain was a wreck and disregarded by most of the world, when she left power Britain was a more successful country. For this great thanks, for this great respect."
Ex Leader of the Liberal Democrats Lord Ashdown has tweeted his response to the news that Margaret Thatcher has died at the age of 87:
Former Liberal Democrat leader, Paddy Ashdown has called on his party to stand begin Nick Clegg, following an attack by Lord Oakeshott.
"When we all overwhelmingly supported Nick Clegg's decision to lead us into government, we knew it would be difficult. Nick challenged us to leave our comfort zone and make the change from a party of perpetual opposition to one capable of carrying the burdens of government.
"The right thing for Liberal Democrats to do now is to continue to do what we have done so well so far. Concentrate on the job we set our hands to under Clegg's leadership. Nothing else."