What’s the difference between a control order and a terrorism prevention and investigation measure? The current equivalent of house arrest has a succinct but sinister-sounding title; its forthcoming replacement, though more explicit, is destined to become a near-acronym (TPIM, pronounced ‘T-pim’) and does not lend itself to the creation of a personal noun such as ‘controlee’.

Apart from the name change, there’s little to choose between them. A control order may be issued, with court approval, if the secretary of state ‘has reasonable grounds for suspecting that the individual is or has been involved in terrorism-related activity’. In the TPIM bill, about to start its passage through the House of Lords, suspicion becomes ‘belief’ - presumably a more demanding test.

Once satisfied, it permits the home secretary to impose a range of restrictions on an individual’s residence, travel, presence in a particular area, movements, bank accounts, ownership of property, communications, association with others and work or studies - to be enforced by reporting, tagging and being photographed.

The control order legislation required annual renewal; TPIM powers will remain available for 10 years. But although individual control orders may be renewed indefinitely, a TPIM may last no more than two years unless it is believed the individual has been involved in ‘new’ terrorism-related activity.

Control orders allow for internal exile: a controlee can be required to live hundreds of miles from his own home. That will not be possible under the TPIM legislation, expected to take effect this year. Nor will daytime curfews.

Perhaps the most interesting difference is that, although controlees can be stopped from making phone calls and using the internet, a TPIM will not prevent an individual using a land-line telephone, a computer with land-line internet access, or a mobile without internet access.

Is this a recognition that access to the internet is virtually a human right these days? Far from it. In his report on counter-terrorism powers published in January, the former director of public prosecutions, Lord Macdonald of River Glaven QC, pointed out that control orders were intrinsically hostile to evidence-gathering. ‘In any other investigative context,’ he explained, ‘the removal of a suspect’s ability to communicate with his co-conspirators on easily penetrated technology would be regarded as bizarre and wholly counter-productive.’

So suspected terrorists will be allowed to make calls and send emails in the hope that they’ll provide evidence on which they and others may be prosecuted. And just in case they haven’t got the message, the legislation will permit the home secretary to order that only phones or computers which have been ‘modified’ by the security service may by used by restricted individuals; that their use of communications is ‘monitored’; and that they may communicate only at specified times - presumably after the monitors have clocked on. You might have thought that nobody subject to a TPIM would dream of communicating, even in code, on a phone or computer issued by the Home Office; but what do we know of such matters?

Despite some differences, control orders and TPIMs both allow the state to impose severe restrictions - based on secret evidence - on individuals who have not been convicted of criminal offences. Successive governments have justified them on the basis that there are a few dangerous people - 12 at the last count - who cannot be prosecuted for lack of evidence but who also cannot be deported, either because they are British or because they would face a real risk of torture abroad.

Is there no way in which we can bring them within the rule of law?

In his report at the beginning of this year, Macdonald said that many of the constitutional objections to detention without trial would ‘fall away’ if the restrictions on individuals were linked to a continuing police investigation. He suggested that an individual should not have his activities restricted unless there was a certificate from the DPP that a criminal investigation into that individual was justified. That would make the restrictions ‘closer in character to bail conditions and therefore inherently less objectionable’.

Macdonald’s recommendation was cited with some approval in a report last week from the House of Lords constitution committee. But I wonder how much difference it would make. The DPP would not find it difficult to confirm that any individual suspected by the home secretary of terrorism justified investigation. As Macdonald says, such people should clearly be the subject of police inquiries in any event.

In the end, the Lords committee could do little more than call for greater vigilance and increased scrutiny. Nobody likes control orders or TPIMs. But most people think we need them.