LONDON (AP) — The carnage in Brussels, accompanied by credible reports about a wider-than-expected network of Islamic State followers active in Europe, has sparked fresh debate in Britain about whether the island nation is made safer or more vulnerable by its European Union membership.

The topic is vital because of the pitched national debate ahead of a June 23 referendum on whether to remain in the EU, a position backed by Prime Minister David Cameron and leading opposition figures, or break away and go it alone, a path favored by London Mayor Boris Johnson, UKIP leader Nigel Farage and others.

The security question has split intelligence chiefs and national leaders, with some arguing that Britain could better secure its borders — and keep jihadis out — if it leaves the EU, while others say Britain benefits by sharing vital intelligence gathered throughout the 28-nation bloc that would be lost if Britain walks away.

Europol director Rob Wainwright said Thursday that Britain got thousands of important leads because of its participation in EU data-sharing and intelligence-sharing protocols. He said leaving the EU would cut off this access at a time when Islamic State extremists pose an "unprecedented" threat to Europe.

He said there had been a "huge" increase in cooperation between European intelligence and police in recent years.

"I see the benefit of that for British police authorities every day," he said, asserting that Britain has access to the names of 300,000 wanted criminals and missing people because of its access to EU data. Removing that access makes no sense, he said.

But former Secret Intelligence Service chief Richard Dearlove disagreed, taking a "Britain first and foremost" view that splitting from the EU would boost Britain's safety by making it far easier to exclude potential jihadis arriving from other European countries.

He downplayed the valued of intelligence-sharing agreements, arguing that Britain has Europe's best intelligence service and "gives far more than it gets" in its dealings with other countries' spy agencies, including many that cannot be trusted with sensitive information because they are riddled with leaks.

The issue has divided the intelligence community. Another former chieftain, David Omand — who headed the GCHQ surveillance agency — said Britain's would be "the loser in security terms" if it leaves the EU.

The issue has also divided Prime Minister David Cameron's Cabinet.