Former chancellor Lord Lawson has dismissed David Cameron's EU re-negotiation as "inconsequential" as the bitter war of words within the Conservatives over Europe continued to escalate, The Press Association reported.
Nigel Lawson, who is president of the Conservatives for Britain group, also poured scorn on a claim by a new grouping of pro-Europe Tories that a vote to leave the EU in the forthcoming referendum would be a "jump into a void".
The increasingly angry exchanges underline the difficulty Mr Cameron faces in holding together his party and his cabinet as his re-negotiation of the UK's membership terms approaches.
The latest spat was sparked by the launch of the new Conservatives for Europe group – headed by former minister Nick Herbert, who led the business campaign to keep Britain out of the euro 15 years ago – to support Mr Cameron's renegotiation.
Writing in The Sunday Telegraph, Mr Herbert warned that leaving the EU would put investment at risk, undermine policing and security, and jeopardise access to European markets.
"Leaving without the first idea of what we might get instead would be to jump into a void," he said.
His comments were dismissed as "complete nonsense" by Lord Lawson.
"It is not a void. It is a place called the world, and we know the world quite well. It is not outer space. We do an enormous amount of business all over the world," he told Sky News's Murnaghan programme.
The former chancellor made clear that he would be voting to leave the EU, whatever deal Mr Cameron was able to get in Brussels. "Even if he gets 100% of what he has asked for it is inconsequential," he said.
Mr Herbert, however, insisted Conservatives for Europe were not "unconditional supporters" of the EU but said that if Mr Cameron – who has given the group his blessing – succeeded in getting what he wanted, it could put Britain in a very strong position.
"Of course I have discussed this with the Prime Minister. The group wants to support his re-negotiation," he told the Murnaghan programme.
"We want reform in the European Union, and provided there is significant reform we would then be willing to say 'yes' when it comes to the choice we should stay in. I don't think we are unconditional supporters of the European Union.
"It is very important know that there is a very substantial body who want to be in a position to say 'yes, we will stay', but only if we get the vital reforms that we need. If we do, Britain could be in a very advantageous position."
The latest interventions come just days after Leader of the Commons, Chris Grayling – an avowed Eurosceptic – said it would be "disastrous" if Britain was to remain in with its current terms of membership.
Mr Cameron has said ministers will be free to campaign on either side in the referendum – but only after he has completed his re-negotiation of British membership terms, which is expected to come to a head at a Brussels summit in February.