Air strikes against Islamic State could last up to three years, according to the Defence Secretary.
MPs have been recalled to parliament to vote on whether to authorise military action against ISIS over Iraq, without putting troops on the ground.
Bombing raids involving RAF planes could begin within hours of a vote in the Commons.
Desperate to avoid a repeat of the Commons defeat over airstrikes against Syria last year, the PM is proposing a cautiously-worded motion intended to win support from all parties.
"This motion does not endorse UK airstrikes in Syria as part of this campaign, and any proposal to do so would be subject to a separate vote in Parliament," it states.
And it adds: "Her Majesty’s Government will not deploy UK troops in ground combat operations."
One woman in St Austell told Pirate FM: "As a mum whose sons have been out to both Iraq and Afghanistan, if anything had been gained I could understand it. But nothing has been gained by it."
Defence Secretary Michael Fallon suggested that the campaign against IS could be a "long haul" of "two to three years".
He told Parliament's House magazine: "(US Secretary of State) John Kerry has estimated two to three years, that looks like a long haul to me.
"But we have to face up to this. This kind of extremism has been spreading, taking root in democracies."
Mike Critchley from Warship World magazine in Liskeard said: "It is a very, very serious situation out there. We all know that our MPs voted against any activity in Syria, but are we going to sit down and do nothing about what is going on? Are we going to sit on our hands? I don't think we should.
"RAF aircraft are already based out in Cyprus working on intelligent-gathering missions over Iraq and that can relatively easily switch to actually firing their weapons.
"They can attack from under the waves too - there is normally one of our submarines in the Gulf."
Poll: Military Action Against Islamic State
Poll closes 26th September 2014 4:45pm.
This poll is now closed.