Powers to allow terrorist suspects to be detained without charge for 42 days are not needed and are unlikely to find favour with the judges who would have to approve them, the Director of Public Prosecutions said yesterday.

Sir Ken Macdonald, QC, who heads the Crown Prosecution Service, said: “In our experience, the 28-day limit works well.” He said that the CPS special counter-terrorism unit was enjoying a 92 per cent conviction rate compared with 77 per cent in other trials, and there was an increase in guilty pleas by terror suspects.

“We have had major, major cases that have occurred within the currency of that time limit, since it was legislated for, and haven't encountered any difficulties in charging those we have wanted to charge and that’s why we have not requested any extension,” he said.

His comments, in an interview with The Times, come as the proposals – contained in the Counter-Terrorism Bill – go before MPs for their second reading today.

Two big terrorism trials are also due to start tomorrow: that of the alleged coconspirators in the 7/7 London suicide bombings of 2005, and that of eight men charged charged over an alleged plot to blow up transatlantic airliners.

Jacqui Smith, the Home Secretary, is appealing to Labour backbenchers to support the measure amid growing criticisms and even a threat of legal action by the Government’s own equality watchdog, the Equality and Human Rights Commission.

Sir Ken, 55, now in his fourth year as DPP, added that the CPS had not requested a period beyond 28 days to give time for charging a suspect, chiefly because in serious cases it already applied a lower so-called threshold test for charging rather than the usual “realistic prospect of conviction”. This lower test enabled suspects to be charged on the basis of “reasonable suspicion” that they had committed an offence and that more evidence would be forthcoming.

In many terrorist cases this test was used and further evidence, sufficient to meet the full test for prosecuting, had always been forthcoming in the following weeks, he said.

“If there was an increase in law to 42 days, and after 26 or 27 days we wanted to extend, and we had to go before a judge, we would be saying: 28 days is not enough even to charge on the basis of a reasonable suspicion of an offence being committed – and we can’t even say there is a realistic prospect of more evidence being forthcoming in a reasonable time. That would be a very challenging application to make.”

Judges, he added, were very independent and strong-minded, and rightly concerned about loss of liberty. “They would want it to take place in very controlled circumstances, particularly precharge.”

Sir Ken, who is credited with turning round the CPS from being the Cin-derella of the criminal justice system to being a key player and top employer, said that the reason for the success of the counter-terrorism unit was its expertise, now unrivalled worldwide.

“In the past 18 months we’ve been building very strong cases, with very good counter-terrorism prosecutors who are expert, working very well with the police. In many cases defendants don’t have anywhere to hide – the case against them is so strong and so compelling.”

The CPS terrorism work was a growth industry and was now among its most important work, along with its twin organised crime division.

“We prosecute more [terrorist] trials than any other fair-trial jurisdiction in the world – more than American, Australia, Canada or other European countries – that’s why our people have become so expert at it.” The rise in guilty pleas was of importance, he added, because it was “a very good way of driving up public confidence”.

“Often, when people are arrested, there are all sorts of allegations of conspiracies, that there is no evidence against them, that particular communities are being targeted, and so on. So when people plead guilty, I think it builds confidence in the process.”

This year so far there have been 11 guilty pleas, 11 convictions and two acquittals – a 46 per cent guilty-plea rate. In 2007, the conviction rate was about the same – 21 guilty pleas, 24 convictions and 4 acquittals – but the guilty-plea rate was lower at 43 per cent.

He does, however, back government plans for postcharge questioning of suspects, which is also contained in the Bill. “I supported this even when at the Bar as a defence lawyer,” he said.

His appointment, in being the first DPP from a radical defence-lawyer background, broke new ground. At the time, the CPS was demoralised, underpaid and the scapegoat for most mistakes in the criminal justice system. Now, he is bullish – with justification – about his record, and even his personal reputation has survived intact despite the blip of bad publicity over his affair with a junior barrister, Kirsty Brimelow.

Two key policies have contributed: the taking over of responsibility for charging from the police, and a push for prosecutors to do their own advocacy, moving into the Crown Court.

In-house advocates were now doing the bulk of organised crime cases in court, he said, and more were being recruited. “They have been startingly successful.” Five had been recruited from the Bar and a sixth was pending. Now the counter-terrorism division would follow suit.

“There’s been an enormous turn-around. We have said we are no longer going to be just a large organisation of instructing solicitors, but of prosecuting advocates. It’s a huge reform and of fundamental importance in recruiting the brightest and best.”

Times Recommends

• Shrinking violet Gordon turns into raging bull
• Eddie Izzard: 'I keep thinking if I do all these things she'll come back'
• Is sex for the disabled the last taboo?