Three young British Muslim would-be suicide bombers were sentenced to life imprisonment this week for plotting to blow up seven airliners over the Atlantic. Directing that they serve minimum terms of up to 40 years, Mr Justice Henriques called the plot the most ‘grave and wicked conspiracy ever proven within this jurisdiction’.

The three terrorists had previously been found guilty of conspiracy to murder. But the jury at their first trial could not agree on a charge that the men had been taking part in a specific plot to blow up planes.

What seems to have persuaded the second jury to convict the three were several coded emails between them and their handlers in Pakistan.

These messages had been intercepted by the security services, but they could not be used in evidence during the first trial because the Regulation of Investigatory Powers Act 2000 bans not only the use of intercepted material in court but even any suggestion that a specific interception has been authorised by a minister.

To get round this ban, British officials obtained a court order in California requiring the internet service provider Yahoo! to hand over the emails. Strictly speaking, even this breaks the 2000 act because it implies that interception has taken place; without it, how would prosecutors have known that the incriminating emails existed?

But let that pass. As far as we can tell, proving the existence of a plot to bring down planes using homemade liquid bombs depended on the use of intercept evidence.

The importance of securing convictions on this charge cannot be overestimated. Without proof of the plot, people would be less willing to accept the security measures that still affect everyone who travels by air; some US airports even screen arriving passengers who need to get through the terminal. More importantly, Britain’s reputation for thwarting terrorism would have suffered a major blow.

So you would think that Britain’s security agencies would now support the use of intercept ‘product’ in court. But all the signs are that they do not.

It is not difficult to see why. The agencies are worried that the use of intercepted material in court would alert terrorists to what the security services can and cannot do. Criminals would be more circumspect if they knew that their communications were monitored.

This is absurd. Everyone knows that emails are not secure. Most corporate emails carry a message saying so. And of course the three terrorists sentenced this week knew this perfectly well – that is why they disguised their emails as innocent, illiterate messages.

But what about other communications – phone calls for example? ‘The existence of an official capability to intercept telephone conversations is not a secret,’ an official report said early last year. ‘However, beyond that basic fact there has never been confirmation of what communications can be intercepted (and how) and what not (and why not).

‘It is often suggested that criminals "know" what the government’s capabilities are,’ the report continued. ‘In reality, they do not know; they often presume based on partial information, experience overseas where conditions are different, and rumour. Some of their presumptions are right, others wrong.’

This comment came from a group of privy counsellors headed by Sir John Chilcot, the former senior civil servant now heading the inquiry into the Iraq war. ‘Many intelligence and law enforcement targets take pains to avoid interception or obfuscate their communications,’ the Chilcot committee explained. ‘At present they cannot know which of their efforts are successful and which not.’

Gordon Brown accepted Chilcot’s proposed safeguards last year, promising parliament that intercept evidence would not be used in court against the wishes of the agency that had originated it.

But that undertaking – and other safeguards – were not enough to see off those who have been calling for the use of intercept evidence in preference to the now-discredited control orders. So Home Office officials spent a year designing ways of using intercept evidence in court before an advisory group, again headed by the ubiquitous Sir John, slipped out a report in February disclosing that the plans had run into problems with the Human Rights Act.

‘There are particular concerns,’ the report said, ‘with respect to continued agency discretion over retention, examination and transcription of intercept material.’ According to the report, ‘there is an estimated core risk of some 40-50% that the regime would be found incompatible with the right to privacy’.

A number of experiments were then held. Sir Paul Kennedy, the former appeal judge and interception of communications commissioner, reported in July that ‘operational live testing of these models took place in March and April 2009 followed by court roleplays during May 2009’. These, he said, ‘highlighted real legal and operational difficulties inherent in using intercept as evidence within the UK’.

Sir Paul could not see a way to overcome these difficulties safely. Clearly reflecting the view of the security services, he called on the government not to permit the use of intercept as evidence unless Chilcot’s original conditions could be met.

The next government should start afresh. As both the former attorney general Lord Goldsmith and the former director of public prosecutions Sir Ken Macdonald argued last week, the question should not be whether the benefits of using intercept outweigh the potential damage. It should be how we can best use intercept evidence in court while minimising the damage to national security.

Preventing terrorism is vital. But so is convicting terrorists.