With the money markets heaping turmoil upon the Government, it is easy to forget that another crisis threatened to bring down the Prime Minister just a few months ago. The Counter-Terrorism Bill, with its controversial proposal to allow the detention of terror suspects for up to 42 days, returns to the House of Lords this week.

As someone who has been deeply involved in every main counter-terrorism investigation since 2005, I am convinced that we will soon need the power to hold suspects for more than the current limit of 28 days - and that we need to legislate for that power now rather than in the middle of an emergency.

But the Government's current proposals are not fit for purpose: they are bureaucratic, convoluted and unworkable. The draftsman's pen has introduced so many hoops to be jumped through that a police case for detaining a terror suspect will become part of the political game.

The debate over detention dates back to 2005. Soon after the July bombings I attended a meeting at the Home Office to review the gaps in police powers to deal with the heightened threat of terrorism. A month later Tony Blair proclaimed that “the rules of the game are changing”. The police - concerned about the increased complexity of terror investigations - asked for more time to keep terror suspects in detention. The threshold of 90 days detention without charge was born.

At that time, I thought the case was strong and self-explanatory. We had endless hours of CCTV to review, mounting numbers of computer hard drives to interrogate, extremely difficult crime scenes to be scientifically examined and much more mobile phone data to analyse. All these things are time-consuming; and time to hold and question suspects was what we were short of.

The debate over 90 days was fierce, the Metropolitan Police got drawn in and we had our fingers badly burnt. The Government lost the 90-day proposal but the detention limit was increased from 14 to 28 days. Given the intensity of that debate, I was astonished when, in July last year, the Government floated the idea of revisiting the detention limit. I remain curious as to what prompted this rethink. Apart from the car bomb incidents at Haymarket and Glasgow airport, nothing had changed since the 90 day defeat. At Scotland Yard, far from trying to rejuvenate the issue we were still bruised from the political mauling of the previous year - why would we want to go there again?

The current Bill, which was heavily amended to secure its passage through the Commons, has some immensely complicated clauses. It would have been my job to make these proposals work but just trying to understand them gives me a headache.

If the Director of Public Prosecutions and the police decide a longer period of detention is needed, the Home Secretary must obtain independent legal advice to satisfy that there is a sufficiently grave threat to justify the use of the 42-day power. Then, the Home Secretary must make a written statement to Parliament. Within seven days, once Parliament has seen the independent legal advice, it must approve or dismiss the request. If approved, a judge has to agree the grounds before issuing a warrant for further detention.

Let's get real. This will just not work. It is an extremely convoluted process that will be unpopular with those who have to make it work. The police and Crown Prosecution Service will have the unenviable task of preparing the necessary papers. Already, the number of police officers taken away from frontline duties to the backroom task of administering control orders has mushroomed; the same will surely occur with this legislative monster. At the height of a terrorist investigation, when lives may be at risk, all efforts should be directed to solving or preventing the crime rather than form-filling or preparing a politically acceptable case for MPs and ministers.

The DPP, an independent lawyer and a judge will all have to decide on the merits of any request for extra detention. But have the politicians considered what the repercussions might be of a dispute between eminent lawyers in the middle of an investigation? And what is wrong with the current arrangement where detention extensions are handled by the DPP and a circuit judge?

MPs will be responsible for gauging the gravity of the threat, with 42 days only being available in “exceptional circumstances”. But the exceptional nature of the situation was never the argument for extending detention limits. The original police case for extra time was because of the complexity and scale of the investigations, but now it may come down to a hair-splitting debate about what constitutes a grave threat to the public. Isn't al-Qaeda terrorism, with its core mission of suicidal mass murder, by definition serious and exceptional?

The passage of this Bill to date has been rancorous. If it comes into law, can we expect MPs to put aside past baggage to dispassionately assess whether an extension is needed in a particular investigation? Even if there was no baggage to set aside, there will still be nagging worries that the process blurs the lines between politics and policing.

The day will come when the current threshold of 28 days will prove insufficient. But this Bill is not the answer. It replaces a legal process that has passed the test of time. The original police case for stronger powers of detention is unrecognisable. It is completely detached from the operational needs of the police. The Bill is about politics and it won't work.

Andy Hayman was Assistant Commissioner for Special Operations at Scotland Yard

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