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.
Any agenda for the future of Europe demands that we increase the cooperation we have with our neighbours - security, strategic surveillance - so that we can stop these criminal gangs that are exploiting the despair of so many young people in the global society today.
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.
Look at what's happening in Europe - the digital and IT industries - we open these up in Europe and British jobs are created. Look at the energy and IT industries - open them up because of our expertise and British jobs are created. There will be 500,000 more high-quality jobs in the next 10 years simply by opening up these markets in Europe where British jobs can do well.
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.