SNP politicians have warned pro-EU campaigners not to embark on “Project Fear Mark II” as politicians and activists gathered in London yesterday morning to launch the official campaign to vote to remain in the EU.

Although the date of the referendum is not yet known, the fight is already under way with two groups opposed to Britain remaining in the EU launching their campaigns last week.

Yesterday, former prime ministers, Tony Blair, Gordon Brown and John Major joined together to endorse the Britain Stronger In Europe campaign.

At the launch in London, the group’s chief executive Will Straw promised to run a positive and pragmatic campaign.

Launching the campaign, ex-Marks and Spencer boss Lord Rose said withdrawing and retreating from Europe is “to misunderstand who we are as a nation.”

“I will not allow anyone to tell me I’m any less British because I believe in the strongest possible Britain for business, for our security and our society,” he said.

The plan for a positive campaign was undermined when Sir Hugh Orde, the former Association of Chief Police Officers president and former chief constable of the Police Service of Northern Ireland warned that the ability of Britain’s police to catch criminals would be limited by a vote to exit the EU.

Orde’s argument was a practical one: “Not only can we [get information about what suspects have done abroad] through what is known as the Association of Chief Police Officers’ criminal records office, we get that information in hours or days, not months and years, so it fits into the judicial process and these people are convicted in the proper way,” he said.

Orde continued, arguing that a No vote would mean Britain having to renegotiate extradition treaties: “So citizens in this country would not face justice. They would not see their people convicted unless we could export, for example, the whole trial to Germany or Poland or France because in the old world they would not let their citizens be tried in other countries.”

He then went on: “My sense is we would go back to that sort of complex, bureaucratic, massively expensive model. And citizens in this country would be put at risk as a consequence. That is not scaremongering. It is simply fact.”

Scottish Government external affairs minister Humza Yousaf tweeted: “ProjectFear Mark II won’t work for EU ref – margins a lot tighter. We should accentuate many positives of reformed EU.”

Stewart McDonald MP tweeted: “If @stronger_in wants to fight on the divisive political ground of British patriotism then I fear it is a fight that it will lose.”

The SNP have distanced themselves from the cross-party group and have made clear they would rather run their own campaign, expressing concerns about being part of a broad coalition involving Labour and the Tories. Sources in the party say they are only too aware of the damage done to Labour after they teamed up with the Conservatives in Better Together.

SNP Europe spokesman Stephen Gethins said: “It is important there is a distinctive Scottish EU referendum campaign – and the SNP will play our part in it – with people from across civic Scotland, trade unions, businesses and political parties who want to remain.”

The board of the Britain Stronger In Europe campaign includes former Chief of the General Staff, Sir Peter Wall, Tory peer and Apprentice star Karren Brady and Innocent drinks co-founder Richard Reed.

Former T4 presenter June Sarpong has the responsibility of energising the youth vote.

Europhile politicians such as Lord Mandelson, Sir Danny Alexander, Damian Green and Green MP Caroline Lucas are also on board.