About time, too, you might say. Pakistan is vulnerable to the charge that it has done too little to curb militant Islam.

Its investigation into the Pakistani links of the 7/7 bombers last year ran into the sands. India blames Pakistan-based terrorist for the Bombay bombings last month, which killed more than 200 people. Militants trekking over Pakistan’s western border are attacking Nato forces in Afghanistan.

All of this adds to the predicament of President Musharraf before next year’s elections.

Too tough on the militants and he may enrage the religious parties, who yesterday teamed up with the main political parties to threaten him with a no-confidence vote.

But too lenient and he would jeopardise relations with the US and Britain. He would also threaten nearly three years of rapprochement with India, which has underpinned recent economic growth, one of the successes of his tenure.

Yesterday the nature of links to Pakistan of the 21 people arrested remained unclear, but officials in Islamabad said that arrests they had made had helped to uncover the plot.

Pakistani families in Britain often have very close links back in Pakistan; if some of the alleged terrorists are from such families then they might too.

Investigators will be trying to establish whether part of the plot was devised within Pakistan, with an active contribution from militants there.

Precedents for this line of inquiry are not encouraging. The police were keen to establish whether the 7/7 bombers who had visited Pakistan before the attacks had help there, possibly from within the madrassas, or religious schools.

But British officials have found those inquiries frustrating. Whatever the blame and counter-blame between Britain and Pakistan on that point, the bottom line is clear: no such role was proven.

Yesterday the Pakistani Government imposed house arrest on the founder and former head of Lashkar-i-Taiba, a militant group that it has already banned, and which India suspects of planning the Bombay bombing. The group was also among those implicated in the December 2001 attack on the Indian Parliament, which brought the two countries to the brink of their fourth war.

Pakistani officials did not link the house arrest to the 21 British arrests, although the events coincided. They have put Hafiz Mohammed Saeed under house arrest before, but then lifted the curb. Saeed resigned five years ago from Lashkar-i-Taiba to become head of a charity called Jamaat-ud-Dawa, widely seen as its sister organisation.

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