Politics is a rough trade, which is why you have to have strong reasons for going into it. Mine were forged in apartheid South Africa. I only played a bit part in the struggle but it was enough to make me want to devote the rest of my life to freedom and justice.

That is why I became an MP; it is also why I am opposed to the government’s Prevention of Terrorism Bill. The home secretary wants to give himself the power to detain and restrict people he thinks, but cannot prove, are involved in terrorism. If he succeeds, he will have abolished an 800-year-old right to freedom that was the envy of the world.

Does it matter? Is house arrest all that bad? It is. I should know. My first husband, Richard Turner, was placed under house arrest by the apartheid government in South Africa in 1973. Finding indefinite imprisonment an expensive way of restraining its opponents, it introduced control, or banning, orders. These confined suspects to specific areas; prevented them from being with more than four other people at a time; removed their passports and forced them to report regularly to the police.

These banning orders had a devastating effect on the life of the suspect and his, or her, family. Richard was a lecturer in political philosophy. He had to stop work.

He could not visit his elderly mother. Nor could he come to Cape Town to see our two daughters. Though divorced some years previously we had remained friends, so I took them to see him. But time spent in his home was marred by the constant surveillance he was under and the surprise police raids. Not to mention the threats and hostility of his white neighbours.

His car tyres were slashed. His brake cables were cut. Wreaths were hung on his front door. Vicious notes were put in his letterbox and threatening telephone calls interrupted almost every night’s sleep.

None of this deterred him. He persisted in the activities that had made him such a threat to the South African state: campaigning for black people to have the right to vote and join trade unions. This is probably why, just before his banning order was due to expire, he was shot by an agent of the South African security service.

It was just after midnight. He died half an hour later in our older daughter’s arms. She was 13. Her sister was nine. They were waiting for me when I flew in from Cape Town the next day. I will never forget the sight, and the smell, of their father’s dried blood in their long blonde hair.

That is when I told them that I was taking them to Britain where people were not detained without trial or put under house arrest. This gave them hope at a time when, and in a country where, people could be detained for 180 days without even being allowed to see a lawyer.

Almost 30 years later — and with Richard’s murder still unpunished — I find it heartbreaking that we are debating a bill that will allow a British home secretary to detain people for 120 days without trial.

Why should this worry you? Nobody could possibly suspect you of terrorism. Don’t be so sure. If this bill goes through, it will allow the home secretary to make decisions about who constitutes a threat to our security in secret. These will be based on intelligence that may not be available to the accused. So you might not even know why a control order is being imposed on you.

The home secretary is a good man for whom I have the utmost respect. But nobody knows how future governments would use an act like this. We can guess at who might have been its victims in the past: Harriet Harman, the solicitor-general, and Patricia Hewitt, the trade and industry secretary, would almost certainly be high on the list. In the 1980s both were considered enough of a threat to have their phones tapped by MI5.

The Prevention of Terrorism Bill allows two types of control order. Curfews and tagging do not require “derogation”, or the setting aside, of the European convention on human rights. House arrest does. The home secretary does not intend to use the derogated powers at present, but he wants to be able to do so.

Is the situation really so desperate that we must trample on our human rights? The fact that Britain is the only Council of Europe member to have derogated from the convention should give us pause. Canada, Sweden and others have managed to restrain the activities of potential terrorists without such infringements of human rights.

The bill had its second reading last week. Tomorrow, it will go through all its remaining stages in 6Å hours. Then it will be sent to the House of Lords. There, it may be changed. I hope it is.

I fear that by introducing such undemocratic means we may, unintentionally, bring about some very undemocratic ends. And what about the effect that passing such a law could have on the reputation of our parliament and democracy? Britain is rightly proud of both.

So, home secretary, please tread softly. This is the stuff our dreams are made of. Destroy them and you destroy us.