Warning: Former defence secretary Liam Fox

Eurosceptics have warned that migrants linked to a wave of sex attacks in Germany will be able to move to Britain if it remains part of the European Union.

Former defence secretary Liam Fox told campaigners the New Year's Eve crime spree in Cologne, which has been blamed on migrants, revealed the 'real security issue at risk' when voters decide if they want to sever the UK's ties with Brussels.

The concerns were echoed by Ukip leader Nigel Farage, who claimed that within three or four years the perpetrators would have German passports that would allow them to head to the UK.

Police in Cologne are investigating a staggering 359 sexual assaults and a total of 821 complaints after a mob went on the rampage on New Year's Eve.

In figures released by the state government in North Rhine-Westphalia, the full extent of crimes - ranging from pick pocketing to gang rape - police are investigating is laid bare in what has been dubbed a 'list of shame'.

It shows that a total of 359 sexual assaults have been reported to German police and that 659 women are recorded as having been victims of the mob.

At the launch of a cross-party campaign on Britain's future in the 28-strong bloc, Dr Fox told the meeting: 'Insecurity for our country comes from open borders and uncontrolled migration.

'I do not want the mistakes made by Angela Merkel in opening the doors to migration in Germany to be reflected in Britain's security, because Germany has discovered in Cologne and other places exactly what it can mean when you do not know who you have allowed into your country.

'Within a short time when they are given citizenship in any of the European countries that they have entered they will have the right to enter the United Kingdom if we remain a member of the European Union.

'That for me is the real security issue at risk in this referendum.'

Liam Fox told campaigners the New Year's Eve crime spree in Cologne (pictured) revealed the 'real security issue at risk' when voters decide if they want to sever the UK's ties with Brussels

Nearly 500 women have made sexual assault allegations following the New Year celebrations in Cologne

At a packed meeting in Northamptonshire, Conservative, Labour, Ukip and Democratic Unionist Party MPs behind Grassroots Out (GO) - a cross party campaign designed to bring together ordinary voters who want to back ‘Brexit' - set out their cases to leave the EU.

It came after David Cameron raised the possibility of Britain getting an 'emergency brake' on high EU migration levels during talks in the Czech Republic on Friday.

But his former leadership rival dismissed the Prime Minister's promise to secure a significant package of reforms to put before voters in a referendum that must be held by the end of next year.

'The very best that the Prime Minister can get in this renegotiation is better membership of the wrong club,' Dr Fox said. 'It's not worthless but it's not a reason to stay win the European Union.'

The GO group, which has been formed in response to frustrations with other anti-Brussels campaigns that are too focused on 'party politics', will set up task forces to knock on doors during the campaign.

Nigel Farage said it was time for the country to 'wake up' to the fact that EU states had been 'forced into a straitjacket of one size fits all'

Established campaigns Vote Leave and Leave.EU have spent months battling it out to win the designation of the Electoral Commission as the official 'out' voice in the referendum on Britain's EU membership.

Labour's Kate Hoey told supporters that 'one of the saddest things' about her party's approach was that leader Jeremy Corbyn and shadow chancellor John McDonnell had failed to publicly maintain their previous opposition to the EU.

Shadow cabinet ministers must be given the same right to campaign freely as the Conservatives, Ms Hoey said.

Mr Farage said it was time for the country to 'wake up' to the fact that EU states had been 'forced into a straitjacket of one size fits all'.