Leading article: Britain's security hinges on curbing terrorism in Pakistan

This week's arrest of several Pakistani students in terror raids across the North West has prompted a fit of introspection about the integrity of our visa system. The Conservatives have demanded that the Government "step up" checks on the roughly 10,000 annual student visa applications from Pakistan. Meanwhile, the Home Office has been talking up its new points-based entry system.

Yet this misses the point. Checks clearly need to be carried out, but we should be wary of the notion that dangerous individuals can be weeded out by a sufficiently rigorous immigration bureaucracy. The history of plots and atrocities in recent years shows us that terrorists do not conform to any simple profiling. The effectiveness of routine background checks is limited.

Terrorists are often discovered to be astonishingly well integrated. An NHS doctor from Iraq was involved in the botched 2007 attempt to bomb a nightclub in London. Mohammed Siddique Khan, the orchestrator of the 7 July London bombing, was a classroom assistant at a Leeds school.

There seems to be an assumption that those individuals arrested in Manchester and Liverpool this week were merely posing as students. But they could easily be genuine. And that would not necessarily make them any less of a threat.

Rather than fretting about visas, we need to get to the roots of the problem: Islamic fundamentalism and terrorism in Pakistan. The UK is getting the blowback from the failure of the Islamabad government to dismantle the terror groups which continue to operate from within Pakistan's borders. Our own security services estimate that about three-quarters of the plots against UK targets they are monitoring have a Pakistan connection.

Pakistan's President, Asif Ali Zardari, has as much reason as anyone in the UK to want his country to be rid of jihadists. His wife, Benazir Bhutto, was assassinated on the orders of Baitullah Mehsud, the leader of one of these groups. Yet since winning power, President Zardari has been more preoccupied with neutralising the threat of his political rival, Nawaz Sharif, than with extinguishing the threat of Pakistan's radicalising madrassas and jihadist groups. There has been little apparent effort either to root out those members of the Pakistani intelligence services who are still offering material support to their old radical clients in the western tribal areas and Kashmir.

The process of crushing jihadist organisations operating in Pakistan will be anything but easy. They have had two decades to entrench themselves. Worse, the tribal regions of Pakistan, where several operate, have historically never truly been under the control of Islamabad. But the longer the delay in taking the fight to these groups, the greater the threat they will pose to Pakistan's very stability. And the greater, too, will be the threat to Britain.

Simply tightening our immigration controls is not going to eliminate the terror threat, not least because we still face a danger from home-grown radicals, many of them British citizens with Pakistani family links. If we stopped issuing visas to Pakistani students tomorrow, that threat would remain.

The forces of terror do not respect national boundaries. And those prepared to murder and die for a perverted interpretation of Islam are not easily identified. We need to wake up to the fact that we will never be able to safeguard Britain's streets totally so long as violent extremism has a base in Pakistan.