Steve Baker, leader of the Conservative for Britain group of MPs, said last month he wanted Mr Cameron to make an announcement on the issue at the party conference in Manchester.
Backbenchers will be allowed a free vote but it remains unclear whether the Prime Minister would suspend collective responsibility like Harold Wilson did to avoid splits in Labour in the 1970s referendum on staying in the Common market.
Mr Cameron said people should not think that his approach to the negotiations meant he was not making headway.
Philip Hammond said the British public would deliver “a raspberry” to the Government if David Cameron’s efforts to renegotiate the relationship with the European Union did not secure “robust” changes.
Defence and security appear set to be at the heart of the conference agenda as the prime minister hopes to capitalise on divisions within the Labour party over Mr Corbyn’s support for unilateral nuclear disarmament.
For at least a generation, Cameron’s party has been riven by a conflict over Europe that contributed to the downfall of both Margaret Thatcher and John Major, the last two Conservative prime ministers.
The Prime Minister again insisted he believed the United Kingdom should remain in a reformed European Union, but stressed: ” I’ve always said if we don’t get those things that we are asking for I rule nothing out and I am very serious about that”.
The main opposition Labour Party swung behind continued European Union membership this week after initial uncertainty about the stance of new leader Jeremy Corbyn.
Lawson, the most senior Tory to join the For Britain group, said it needed to get its campaign under way. But Mr Cameron says today that Vladimir Putin’s decision to start conducting air strikes in Syria will not stop British drones conducting missions there.
“But remember, the PM has also said nothing is off the table”.
As well as increased recruitment if it is requested, the SAS and other elite forces will be given new equipment and weapons to ensure that they are “properly beefed up”.
Setting out the new drones being made available to deal with IS, Mr Cameron told the Sunday Telegraph: “We have at the moment a drone fleet of 10 Reapers and what we are going to be doing is actually replacing that with twice as many with a new updated piece of equipment – called Protector – which will be more than doubling our fleet to keep us safe and to give us the intelligence and information and potentially give us the capacity to hit people who are potentially planning to hit us”.
“If you want to be part of the government you have to take the view that we are engaged in a renegotiation to have a referendum that will lead to a successful outcome”, he said in June. “We try to take every step we can but at the end of the day we have to keep the British people safe from terrorist threats”.