LONDON, Sept 10 (Reuters) - The Taliban takeover of Afghanistan will provide a "morale boost" to extremists plotting attacks elsewhere, and could again give them a base to operate as they did in the run-up to 9/11, the head of Britain"s MI5 domestic spy service said on Friday. Ken McCallum, director general of the Security Service, best known by the initials for Military Intelligence Section 5, told the BBC the threat to Britain from terrorism was "a real and enduring thing". "We do face a consistent global struggle to defeat extremism and to guard against terrorism," McCallum said in an interview on the eve of the anniversary of the Sept. 11, 2001 attacks on the United States. Britain last faced major attacks in 2017, when a bomber struck a concert in Manchester and knife-wielding men attacked two bridges in London. In the four years since, police and intelligence services had disrupted 31 late-stage plots to attack Britain, McCallum said. Such militants would be inspired by the Taliban success. "Overnight you can have a psychological boost, a morale boost to extremists already here or in other countries, so we need to be vigilant," McCallum said. "There is no doubt that the recent events in Afghanistan will have heartened and emboldened some of those extremists." The Taliban have promised they will not let Afghanistan, where Osama bin Laden plotted the 9/11 attacks when they were last in power, again become a haven for militants planning to strike the West. But McCallum said there was a risk that this would be exactly what happens. "Alongside the immediate inspirational effect is the risk that terrorists reconstitute and once again pose us more in the way of well-developed sophisticated plots of the sort that we faced in 9/11 and the years thereafter."