Terrorism cannot be solved without cooperation across Europe, Gordon Brown has said.
In an impassioned speech at De Montfort University in Leicester, the former prime minister insisted that Britain working with its European neighbours, as well as countries in Africa and the Middle East, is the only way to tackle the causes of terrorism.
"Only the European Union coming together can offer the diplomatic help, the aid, the economic incentives, the strategic support, so we can deal with the causes of terrorism," Mr Brown said.
The leading Vote Leave campaigner also insisted that the only way illegal immigration masterminded by people traffickers can be tackled is by cooperating with countries across Europe.
Mr Brown also hailed the EU as instrumental in helping to meet energy targets, guaranteeing minimum standards to protect workers and safeguarding - as well as creating - up to half a million British jobs.
Mr Brown also said the EU could be used to set up a register of offshore trusts and businesses using tax havens, and levying sanctions against non-complying companies.
"You cannot do that just on your own," he said. "You need the collective weight and the strength in numbers that comes from the European Union.