If you ask Google “What is the meaning of life, the Universe and everything?” the search engine replies: “42”. This is, of course, the response given by the Deep Thought Computer in Douglas Adams's The Hitchhiker's Guide to the Galaxy. But it could just as easily explain why our Deep Thought Prime Minister decided that it should be possible for the State to detain terrorist suspects without charge for 42 days.

The number on which Gordon Brown's reputation now depends was plucked randomly from the air. I clearly remember conversations with ministers just a few months ago in which they admitted that they had absolutely no idea how long the detention period should be. Even Jacqui Smith, the Home Secretary, made clear to begin with that the Government was not remotely committed to 42 days. Now, however, the number has become a totem by which Mr Brown's leadership will be judged.

The Labour whips have gone into overdrive in an attempt to win tomorrow's vote. Leftwingers are being advised that if they oppose Mr Brown they will get the Blairite David Miliband in his place. The “walking wounded”, those who in normal circumstances would be deemed too sick to attend the House, have been told they must hobble through the division lobbies.

Ministers have had to cancel foreign trips. Gordon Brown has been making more of his infamous 6am calls to opponents of the measure - yesterday he was ringing the serial rebels who will never fall into line, a sign of how desperate the Government is. There are even rumours that Labour tried to buy off Shami Chakrabarti, the director of Liberty, with the offer of a peerage.

This is no longer about policy, it is about politics. The Prime Minister wants to look strong. He is determined to position himself as tough on terrorism, in contrast to the lily-livered, parting-swapping David Cameron. The problem for Mr Brown, however, is that, having made so many concessions to his opponents, he risks looking confused about what he really believes. Even if he wins the vote, a measure designed to demonstrate his strength on crime could end up proving the weakness of his political authority.

The package drawn up by the Government to reassure Labour rebels has created a constitutional minefield. Parliament is to be given a vote within seven days on whether a terrorist suspect should be held for the extended period - but this is as nonsensical as Alice in Wonderland's Rule 42, which states that anyone taller than a mile must leave the court. Either MPs will be able to debate only broad generalities (in which case the safeguard of parliamentary scrutiny is meaningless) or they will discuss detailed allegations (in which case it would be impossible for there to be a fair trial). And what if the suspect appeals - as he almost certainly would - against the Home Secretary's decision to ask for a longer period of detention?

A judge could rule against the extension at the very moment that the Commons was giving it the nod. Then whose verdict would come out on top? Parliament, elected to represent the voters, or the court, appointed to protect the voters from an over-powerful executive? No wonder the “Establishment” - the security services, police and prosecutors, as well as MPs, are divided in a way that rarely happens when the safety of the nation is at stake. I have lost count of the number of ministers who have told me that they wish Mr Brown would abandon the 42 days proposal.

“The whole thing is totally unworkable,” one said.

“He should concentrate on the things that matter to the voters,” argued another.

“It's not worth the political capital he's expending on it,” added a third.

The Government's law officers, the Attorney-General and the Solicitor-General have expressed reservations to No10. Jack Straw, the Justice Secretary, had early concerns and, according to some insiders, even Ms Smith has not always been as enthusiastic in private as she is in public about the plan. Civil servants are divided, with an institutional split between Home Office officials, who support detention without trial, and Justice Department mandarins who oppose it on civil liberty grounds. As one minister put it: “This is all being driven by No 10.”

The Prime Minister's allies argue that the “42-days vote” will give him definition as a “security leader”. But Mr Brown is not willing to go for broke as the scourge of terrorists. He would never declare - as Tony Blair did after the attacks on London - that the “rules of the game have changed”. In fact, he has deliberately distanced himself from his predecessor by emphasising a greater commitment to civil liberties. Last year he made a speech in which he quoted Orwell, Milton and Locke to make the case that liberty was Britain's “gift to the world”.

“Instead of invoking the unique nature of the threats we face today as a reason for relinquishing our historical attachment to British liberty,” he said, “we must meet these tests not by abandoning principles of liberty but by giving them new life.” The proposal to bang people up for six weeks without charge is more like the kiss of death.

There is the same ambiguity in law and order policy more generally - one day Mr Brown wants more people locked up in “titan prisons”, the next day he thinks there should be an increase in community sentences. One MP close to the Prime Minister says that Mr Brown coined the phrase “tough on crime, tough on the causes of crime” and that unlike Mr Blair “he believes both bits”. But on terrorism Mr Brown cannot face both ways. Tough or tender, which is it?

In Japan, the number 42 is considered bad luck because the words 4 and 2 pronounced together mean “death”. Even if Mr Brown wins the 42 days battle this week, he is in danger of losing the war to tell the voters who he really is.

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