Boris Johnson was wrong to rule out Britain being able to remain in the EU even if the country votes to leave in June, former Tory leader Michael Howard has said.

The mayor of London was forced to declare that “out is out” a week after appearing to keep open the option of a Leave vote resulting in fresh negotiations with Brussels and a second referendum on a better membership deal.

David Cameron has dismissed the idea – seen by some Out campaigners as a key weapon to reassure uncertain voters leaning towards quitting the 28-nation bloc – as a “complete fiction”.

Lord Howard on David Cameron

He has been backed by the Former European Commission president Jose Manuel Barroso who said anything other than an EU exit after a Leave vote was “unthinkable … practically impossible”.

But Lord Howard said the UK would be so “sorely missed” by the rest of the EU that other leaders could offer more compromises.

Asked about Mr Johnson’s clarification, he told BBC1’s Sunday Politics: “I don’t agree with him. The European Union has form on this: it has done it before in relation to Ireland, in relation to Denmark.

“The very things that make it certain that we would thrive as an independent country – the fact that we are the fifth biggest economy in the world, the strongest military power in Europe, the fact that we are the second biggest contributor to the European Union’s budget – those things would mean we would be sorely missed if we did leave.

“That is why I think the EU’s leaders would say, if we did vote to leave, ‘let’s have some more talks’.

“I can’t guarantee that would happen, but it is a possibility.

“If they don’t come back. If all we are left with is the current, unreformed European Union, I think we are better out than in.”

What has the EU ever done for us? Show all 7 1 /7 What has the EU ever done for us? What has the EU ever done for us? 1. It gives you freedom to live, work and retire anywhere in Europe As a member of the EU, UK citizens benefit from freedom of movement across the continent. Considered one of the so-called four pillars of the European Union, this freedom allows all EU citizens to live, work and travel in other member states. What has the EU ever done for us? 2. It sustains millions of jobs A report by the Centre for Economics and Business Research, released in October 2015, suggested 3.1 million British jobs were linked to the UK’s exports to the EU. What has the EU ever done for us? 3. Your holiday is much easier - and safer Freedom to travel is one of the most exercised benefits of EU membership, with Britons having made 31 million visits to the EU in 2014 alone. But a lot of the benefits of being an EU citizen are either taken for granted or go unnoticed. What has the EU ever done for us? 4. It means you're less likely to get ripped off Consumer protection is a key benefit of the EU’s single market, and ensures members of the British public receive equal consumer rights when shopping anywhere in Europe. What has the EU ever done for us? 5. It offers greater protection from terrorists, paedophiles, people traffickers and cyber-crime Another example of a lesser-known advantage of EU membership is the benefit of cross-country coordination and cooperation in the fight against crime. What has the EU ever done for us? 6. Our businesses depend on it According to 71% of all members of the Confederation of British Influence (CBI), and 67 per cent of small and medium-sized enterprises (SMEs), the EU has had an overall positive impact on their business. What has the EU ever done for us? 7. We have greater influence Robin Niblett, Director of think-tank Chatham House, stated in a report published last year: “For a mid-sized country like the UK, which will never again be economically dominant either globally or regionally, and whose diplomatic and military resources are declining in relative terms, being a major player in a strong regional institution can offer a critical lever for international influence.

Lord Howard – a mentor to Mr Cameron, who served as his special adviser early in his political career, said telling the PM he was backing the other side had been “very difficult”.

“We had a conversation which was a difficult conversation,” he said.