South Africa is the richest, most powerful and most developed country in Africa. It has a strong historical association with Britain, the former ruler whose relationship with its imperial possession, over the past century and through two world wars, was long seared by a brutal colonial war. South Africa has since emerged from apartheid-era isolation to rekindle trading, investment, sporting and cultural links with Britain. It has also, unfortunately, become a backdoor for thousands of criminals, terrorists and illegal immigrants to smuggle themselves into this country. It is time that door was firmly closed.

Law enforcement agencies are now urging the Government to change the immigration rules which allow South African passport holders to enter Britain without a visa and stay for six months. Too many people have abused this recently granted freedom. One people-smuggling gang broken up by the Serious Organised Crime Agency is suspected of having brought more than 6,000 illegal Indian immigrants to Britain on forged or stolen South African passports. British intelligence is also concerned at the increasing number of al-Qaeda terrorists, trained in Pakistan, who use South Africa as a stopping-off point to thwart the tighter checks and controls of those moving directly between Britain and Pakistan. In South Africa the terrorists can lie low among the large Asian community, before travelling on to Britain months later on a forged passport and under an assumed name. And the South African police and intelligence services have been too preoccupied with the domestic crime wave to pay much attention to this threat.

Imposing a new visa regime is a drastic step. It will severely inconvenience the 450,000 South Africans who come to Britain each year, including some 180,000 tourists. It will also handicap the growth of business links, making it harder for Britain to retain its position as one of South Africa's leading trade partners. If each traveller needs a pre-approved visa, far fewer than the 57,000 South African transit passengers arriving in 2006 will pass through London in future. And the 1,450 students, who already need visas for a long-term stay, may also feel less welcome.

Visas are imposed only with reluctance, and usually in response to a short-term problem. Britain reinstituted a visa requirement for Turkey and Sri Lanka in response to a rapid rise in migration in the 1980s, and visas were mooted in 1999 for all Czech visitors after a sudden influx of gypsies seeking political asylum. These steps caused considerable annoyance, and the Turks responded swiftly with a reciprocal demand. But British attempts to abolish mutual visa requirements for countries beyond Europe have largely been halted, as fears grow that this will only encourage illegal immigration. Russia has repeatedly lobbied the European Union to drop visas for Russian tourists, without success. Given the present failure of recent governments either to cut the migrant inflow or to guard against the infiltration of terrorists posing as asylum seekers, the South African loophole is one that ministers will be under considerable pressure to close.

South Africa could and should take measures to avert such a step. Police and intelligence agencies already have channels of co-operation. They must be used by Pretoria more effectively. No one would welcome hampering links with Britain's leading political and economic partner in Africa. But these links must reinforce, not jeopardise, Britain's prosperity and security.