There will be much talk tomorrow about the big decisions George Osborne faces over the future of the British economy, as he prepares to deliver his budget. But 100 days from now, a decision of far greater economic consequence awaits all of us.

EU referendum debate – share your reaction | Sarah Marsh Read more

On 23 June, we face the biggest decision any of us will ever make about the future of our economy and our country – leave, or remain. It is a choice that will have profound implications for jobs, growth and living standards. Our children and our grandchildren will live with the consequences.

As we enter what is in danger of being Britain’s final 100 days as a committed member of the European Union, the arguments being made by the leave campaigns have become increasingly muddled. Nigel Farage seems to want to take us back to an imaginary golden age.

And having struggled with an “agonisingly difficult” decision only last month over whether to back Brexit, Boris Johnson now seems to see no dangers whatsoever in leaving, and says doing so would lead to a “bright, bright future”. What exactly were you agonising over then, Boris?

Clearly, there is some deception going on. Johnson, Farage and the other Brexiters are quick to denounce anyone who spells out the facts about what Brexit would mean for Britain’s prosperity, security or influence as constituting “Project Fear”. But it is increasingly clear that the leave campaign is the one that is reliant upon deception – let’s call it Project Fantasy.

Project Fantasy goes like this. Diminish Britain’s standing by suggesting that Europe is somehow something that is done to us; ignore the ways in which, from terrorism to climate change, we are far more effective working with our partners; label all workers’ protections “red tape” and say the economy would be better off if we could scrap them; ignore the huge economic benefits our EU membership has brought us and try to convince people that there is a land of milk and honey waiting outside the EU without spelling out how to get there.

Brexit would cause chaos, confusion and economic uncertainty

Having seemingly given up on arguing that Britain could gain access to the single market under terms similar to Norway or Switzerland, but somehow pay less into the EU budget and not be required to implement the same rules (including free movement) the Brexiters are now pinning their hopes on what Johnson calls a Canada-style free trade approach. To argue that this would be an improvement on what we have now is absurd; Canada’s free trade agreement with the EU has taken seven years to negotiate and still doesn’t cover all goods and services. That would be a bad deal for Britain. It is also implausible that the UK could strike better trade deals with third countries than with the EU. As the US trade negotiator Mike Froman has said, the US is “not particularly in the market” for a trade agreement with a single country such as the UK.

But the biggest fantasy of all is to deny the huge damage that leaving the largest single market in the world would do to our economy. Look at the facts. Almost half of our exports go to EU countries. Over 200,000 British companies export to the EU. We receive an average of £26.5bn of investment every year from other EU countries. So it’s little wonder that the message from businesses of all sizes across the country is so resounding – a poll published today by the CBI shows 80% of their members think being in the EU is best for their business.

Companies from all over the world choose to build offices and factories in the UK, and recruit staff here, because we are a gateway to the European single market. And though it is difficult to quantify precisely, somewhere in the region of three million jobs are linked to our membership of the EU.

According to Project Fantasy, these are marginal concerns. But for the Labour party people’s jobs, livelihoods and living standards, not to mention workers’ rights secured by our EU membership, are absolutely fundamental concerns.

That’s why the decision we take on 23 June is so profound. The stakes couldn’t be higher. Brexit would cause chaos, confusion and economic uncertainty as Britain spends time and treasure frantically trying to get back to the advantageous position we are in now. Project Fantasy would dissolve within the first 100 days as it comes into contact with cold reality.

• The Guardian Live EU referendum debate featuring Alan Johnson, Nick Clegg and Nigel Farage is tonight at 8pm. It will be live streamed on the Members’ homepage