Boris Johnson has branded the euro a "calamitous" project, calling for Britain's relationship with the European Union to be pared back to the single market and put to a referendum.

Pre-empting a speech by David Cameron on Europe, to be delivered before Christmas, the Conservative London mayor outlined his own "minimalist strategy" which would see Britain shift to an "outer-tier" relationship with Europe focused on trade.

But he said that to reach such a "nirvana" the government would need to change its tactics, delivering a veiled swipe to the prime minister and George Osborne by criticising those who have supported closer fiscal union within the eurozone.

Cameron has said he supports the idea of a referendum, although he has not confirmed any details or said what question he would favour. Johnson said that favoured a renegotiation, followed by a referendum with a clear in-or-out question. He said he narrowly favoured staying in the EU, assuming Britain's relationship with it changed, and spelled out his own vision of a relationship boiled down to the single market.

"There's a perfectly viable relationship to be had which is happy, contented with the single market, trading freely but not with the whole shebang," Johnson said during a speech at the Thomson Reuters headquarters in Canary Wharf.

The mayor told an audience of leading City figures, ambassadors and diplomats that the euro had been a "calamitous" project that would limp on and eventually "blow up", though he resisted predicting when that might be.

"I don't understand why we continually urge the eurozone countries to go forward with this fiscal and political union, when we know in our hearts that it is anti-democratic and therefore intellectually and morally wrong," he said.

Cameron and Osborne have both given their support for closer fiscal union within the eurozone.

Johnson said: "We can no longer pretend that this country is at the heart of Europe" when EU politics were increasingly focused on projects to establish closer union between the eurozone states.

The situation was liberating as it allowed the UK to "seize the moment to ask the British people to define themselves and their future in Europe".

Britain should demand a "commonsense" relationship, with the UK involved in the single market and the decisions governing it, but preserving its freedom to set its own monetary policy, interest rates and taxes, while having less bureaucracy and fewer intrusions from Brussels.

Such a relationship was both essential and deliverable, said Johnson, who added that voters could opt to leave the EU altogether if they did not like it.

"We should use the opportunity of the treaty changes – perhaps over the banking union – to convene an inter-governmental conference in which we bring Britain's membership in line with what people want.

"Boil it down to the single market. Scrap the social chapter. Scrap the fisheries policy. We could construct a relationship with the EU that more closely resembled that of Norway or Switzerland – except that we would be inside the single market council, and able to shape legislation.

"That is a renegotiated treaty we could and should put to the vote of the British people. It is high time that we had a referendum, and it would be a very simple question: 'Do you want to stay in the EU single market – yes or no? And if people don't think the new relationship is an improvement then they will exercise their sovereign right to leave the EU."

Johnson, a former Brussels correspondent, said during a question-and-answer session that his views on the EU had moved from being quite enthusiastic to "really very sceptical", but he said his support weighed in favour of staying within the EU.

"Whenever I sat down and put a cold towel around my head and I totted up the pros and cons of us getting out and seeking a glorious independent future on the high seas and that kind of thing, or staying in, I always came down in favour narrowly of staying in and I still narrowly think that is the right way."

Johnson warned that many were seeing Britain "slowly sliding towards the exit", which he said would be welcomed by some within the EU and many in the UK.

He believed there would be support from other member states for a renegotiated relationship between the UK and the EU, who would "welcome a resolution of the British question".

"I think it's one that should work, and if you look at the choice that other [EU] countries are going to face –do we lose Britain altogether? Do we lose access to their markets? Do we lose any kind of relationship with Britain or do we keep them broadly in the single market on the terms that they want? – I think they will go for it."

Johnson rejected warnings that a renegotiated relationship with the EU would turn the UK into a backwater, insisting that London would remain at the heart of the world economy, trading freely not only with Europe but also the emerging economic giants, such as India and China.