David Cameron will reportedly warn the British public that a breakaway from Brussels would not lead to a "land of milk and honey" when he delivers a speech at the Northern Future Forum in Iceland on 28 October. The biblical reference, from the book of Exodus, will mark a shift in tone for the prime minister as he starts to campaign against a Brexit.

The Conservative leader is expected to meet the leaders of Denmark, Estonia, Finland, Iceland, Latvia, Lithuania, Norway and Sweden at the summit. Iceland and Norway are members of the European Economic Area (EEA), rather than the EU, but the prime minister will argue that this constitutional arrangement would not suit Britain.

A government source told the Daily Telegraph that Cameron would warn: "It's not necessarily a land of milk and honey. People need to understand there are significant downsides [of a Brexit]." The prime minister will add: "Norway has to pay hundreds of millions of euros in membership fee to access the single market."

But Cameron still has not revealed when the UK's historic referendum on its membership of the EU will be held. The only promise that the prime minister has made on the date is that the vote will be before the end of 2017 as he continues to negotiate with Brussels over welfare and other reforms.

The latest opinion poll from ICM, of more than 2,000 people between 23 and 25 October, put Remain seven points ahead of Leave (45% versus 38%, respectively).

Meanwhile, Ukip leader Nigel Farage has mocked Cameron's decision to campaign against a Brexit while in Iceland. The Eurosceptic firebrand took to Twitter to argue that Iceland has "benefited hugely" from being outside the political and economic union.