Britain geared up for renewed military action in Iraq with lawmakers expected to vote in favour of joining air strikes against Islamic State (IS) militants Friday despite echoes of the unpopular US-led 2003 invasion under Tony Blair.

Prime Minister David Cameron will kick off the debate at the House of Commons from 0930 GMT and has argued that Britain should not be "frozen with fear" over fresh military action in Iraq.

Six British Tornado fighter jets based in Cyprus are poised to begin raids on IS within days or even hours, beginning a military campaign that Defence Secretary Michael Fallon has said could last three years.

If the vote is passed, Britain would join the US and France in launching targeted strikes on the IS group in Iraq, where it controls swathes of territory, as in neighbouring Syria.

IS fighters have beheaded a British aid worker, David Haines and two US journalists, and are holding two other Britons, Alan Henning and John Cantlie.

Britain will not as yet join US-led air strikes on Syria, which are backed by five Arab states. Cameron's government says a separate parliamentary vote would be needed for that to happen.

Foreign Secretary Philip Hammond said that Britain was prepared to fight the IS group until "they no longer pose a real threat".

"We judge that ISIL (another name for IS) does pose a serious threat to our security and we will continue to deal with ISIL... until such time as we judge that it does not pose such a threat," he told BBC radio.

Some lawmakers are expected to oppose military action because of fears that the mission is ill-defined, fuelled by memories of Britain's role in the 2003 US-led invasion of Iraq.

Between 2003 and 2009, 179 British personnel died in Iraq and the last British troops only left the country in 2011.

"I believe there's a big danger of mission creep with this and no-one can tell us what the endgame is," Labour lawmaker Diane Abbott told Sky News.

"We spent £8.4 billion (10.7 billion euros, $13.7 billion) on the last Iraq war and Iraq is even more divided, more murderous, more riddled with terrorists than ever."

Blair, prime minister when Britain went to war in Afghanistan in 2001 and Iraq in 2003, has urged Cameron not to rule out sending ground troops again "if it is absolutely necessary."

The Stop the War Coalition -- which helped organise a million-strong demonstration against the 2003 Iraq war -- staged a protest of around 200 people Thursday and has vowed further demonstrations if the vote passes.

Hostage safety concerns

Mindful of a war-weary public and a damaging parliamentary defeat last year over military action against President Bashar al-Assad's regime in Syria, Cameron's government has prepared the ground carefully for Friday's vote.

The parliamentary motion, released in advance, stresses that the government will not deploy ground forces and that Iraq's government had requested assistance to fight the IS group, making it legal under international law.

Cameron has also argued for weeks that the "murderous plans" of extremists pose a direct threat to British people.

On Thursday, police in London arrested nine people suspected of extremist Islamist links and another two were held Friday.

Officials believe 500 Britons have travelled to fight with IS.

A militant with a British accent who is thought to have killed Haines and two US journalists has been identified, FBI chief James Comey said Thursday, without revealing the man's identity.

The family of Henning, a taxi driver captured while delivering aid in Syria, has voiced fears that air strikes against IS group targets could put his safe return in jeopardy.

"If they're going to do air strikes on them, they'll just run away. They'll take him with them and no-one will know where he is again," his brother-in-law Colin Livesey told ITV television this week.

Hammond said that British hostages were at risk regardless of whether the country joined air strikes.

"It is not the case that these hostages are safe if we don't go in and unsafe if we do go in," he told Sky News. "Their safety is hugely threatened because of the nature of what ISIL is".