10/11/2015 12:41 GMT | Updated 10/11/2016 05:12 GMT

For Those Calling for Britain to Leave the EU the Question Is Clear - Why Would We Put Our Success at Risk?

This morning at Chatham House, the Prime Minister set out a series of sensible and sound reforms to improve Britain's relationship with the EU. In doing so he spelled out the big question facing the British people: would the UK would be stronger, safer and better off within Europe or out on our own.

This - as the Prime Minister said today - is the biggest choice facing our country in a generation. And the stakes are high. Our prosperity, jobs and security are all at risk.

What the British people deserve, and will demand, in the coming months is a vigorous debate in which both sides set out their case. But what we are already seeing from Leave campaigners is an attempt to intimidate legitimate voices in this debate. It was extraordinary to hear the head of the Vote Leave campaign boasting about how 'nasty' their campaigning would get.

'Project Nasty' is the Outers way of silencing debate because they are already losing the argument. So when British businesses - big and small - repeatedly say that they want to remain in the EU - not least because it is best for jobs and investment - the Out campaign tries to shut them up.

Intimidating letters, threats to disrupt company AGMs, and co-opting students to do their dirty work - theses aren't the tactics of a campaign with the confidence that it is winning the big arguments. These are the desperate acts of side that has seen its case fall apart.

Just look at the mess they get themselves into when asked the simple question, 'What would leaving the EU actually mean for British trade?' For a while it was the 'Norway Model' which would mean paying into the EU budget with no say over the rules and regulations. When the Prime Minister and Norway's own foreign minister exposed the problems with that approach, we almost heard a different answer from every eurosceptic as they sought refuge in Iceland, Switzerland, or even the World Trade Organisation which has not negotiated a successful trade deal in almost 20 years.

What has been absolutely clear in recent weeks is that when Leave campaigners are forced to discuss a future outside the EU their arguments fall apart. Just this morning, the director of one of the leave campaigns called for an individual trade agreement with the US, blissfully unaware that the Americans have already made clear they have no interest in pursuing one.

This is why the Leave campaigns are in crisis. Underneath the bluster, they do not have a shred of evidence that Britain would be better off outside the EU.

What we know today is that Britain enjoys tremendous benefits from our place inside Europe. Three million jobs in Britain are linked to our trade with the rest of Europe, and our free trade with the world's largest single market drives down the price of goods and services.

In fact the CBI estimates that all the trade, investment, jobs and lower prices that come from our economic partnership with Europe is worth £3,000 per year to the average household. That's a return on investment for our contribution to the EU budget of almost ten to one.

As academics at the LSE have shown, governments over many years have excelled at getting the best from Europe for British citizens. Anyone pretending otherwise is talking down Britain.

When we calmly assess the evidence it is clear that benefits of our place in Europe clearly outweigh the costs. Of course not everything in perfect which is why the Prime Minister is seeking his ambitious reforms. But Prime Ministers in the past from Harold Wilson to Margaret Thatcher to Tony Blair have gone to Brussels and got the best back for Britain.

As David Cameron has made clear, the UK is negotiating from a position of strength. We are the fifth largest economy in the world, we are growing faster than anyone else in Europe and we are part of the largest free trade area in the world. So for those calling for Britain to leave the EU the question is clear - why would we put that success at risk?

Will Straw is the executive director of Britain Stronger In Europe