Yesterday the Cambridge Union was due to play host to Mitchel McLaughlin of Sinn Fein. But Ged Martin points out that it is not easy stopping students debating politically sensitive topics. Cambridge University tried in 1817 - and failed.

The Government does not like student unions to meddle in politics. Tories yearn for the days when the thunderous disapproval of a vice chancellor was enough to frighten undergraduates back to their books. Alas, even in the good old days it did not work.

The story begins in 1814, when a group of old Etonians engineered the merger of three existing clubs at Cambridge University into a "union''.

The new union - "one mass of noisy ignorance'' as a detractor called it - was housed in the back of a local pub. On February 20, 1815, the Cambridge Union held its first debate, condemning by two votes the refusal of the Whigs to form a coalition with Lord Liverpool's Tories in 1812. For the next two years, the union pronounced youthful verdicts on personalities as diverse as Burke and Mahomet. Unfortunately, the political climate was freezing fast. Early in 1817, scared by postwar revolutionary unrest, ministers suspended habeas corpus. The Cambridge Union approved - by just one vote. Worse still, the new radical movement was organised by the eccentric Major Cartwright into bodies called "unions". For Cambridge's vice chancellor, James Wood, the flourishing debating club behind the Red Lion was an embarrassment, both in its existence and in its name.

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Yet like a good academic bureaucrat, Jem Wood did not wish to be seen to be bowing to political pressure. Happily, an undergraduate who hated student politicians solved his problem by turning vice chancellor's evidence at a crucial moment. Wood claimed to have received a letter from a student who alleged that his studies had been harmed and his prospects in life blighted by time wasted at union debates.

This gave Wood the excuse he needed: the union could be suppressed on educational rather than political grounds. On March 24, 1817, as the union was starting to debate the dangerous question of the role of the army in society, the university's own police - the proctors - ordered the meeting to disperse.

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Yet four years later, the Cambridge Union was back in business. Wood's successor, Christopher Wordsworth, brother of the poet, permitted the resumption of debates in 1821. Why? Because the union made a deal. First, "no discussion whatever shall take place on a motion for the admission or rejection of any publication". Members had been using business meetings discussing the purchase or cancellation of newspapers to deliver speeches on the politics of those publications. It would be safer to have the young gentlemen openly debating than secretly making inflammatory political speeches.

Hence rule change number two: no political topic could be debated "subsequent to the year 1800''. On March 6, 1821, debates resumed with a decorous decision to approve American resistance to George III.

Yet union bosses had a struggle to make the deal stick. Wilder spirits would insist on dragging in post-1800 considerations to illustrate their pre-1800 arguments. A further amendment to the rules was hastily proposed: nobody was allowed to mention any event after 1800. The 1800 rule was none the less a farce. The union voted overwhelmingly in 1824 that the Greeks should have fought for independence in 1799. The only problem was that the Greeks had not fought in 1799 but were doing so in 1824.

Gradually, the 1800 barrier was replaced by a sliding 20-year rule. Even this vanished during the excitement of Wellington's resignation in 1830. It would have been hard to argue for or against Wellington as prime minister 20 years earlier. Despite repression by the authorities, the world's prototype student union had broken free of political control.

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Were the interests of the ancient University of Cambridge threatened by undergraduate free speech? Hardly. In 1834, the union resolved that it would not be "expedient'' for the fledgling University of London to grant degrees. Even old Jem Wood would have supported that.

Ged Martin was president ofthe Cambridge Union in 1968 and is now director of the centre for Canadian studies, University of Edinburgh.