By Laura Kuenssberg
BBC political correspondent
An imperative legal change to confront the growing threat of terrorism, or an unnecessary attack on the fundamental principles of the law?
The government is risking defeat in parliament over the 42-day move
Home Secretary Jacqui Smith has again made clear her determination to push through legislation that would allow the police to hold terror suspects for up to 42 days without charge.
Such cases would be monitored by a judge, and parliament would have to give its permission within 30 days.
In an interview with a Sunday newspaper, Ms Smith said terror plots are becoming more complex, so it takes more time to gather evidence required to charge a suspect.
Her cabinet colleague, Alan Johnson, told BBC1's Politics Show that 42 days would only be used in the rarest of cases.
"We would move beyond 28 days on the recommendations of the police, on the recommendations of the judge...they'll be in very few and exceptional cases but I think that's necessary given the complexity of the plots we're trying to crack," he said.
But the reality for ministers is that dozens of Labour MPs are opposed to the legislation - agreeing with the Conservatives and the Liberal Democrats that ministers have not yet presented enough evidence to warrant the change.
Shadow security minister, Dame Pauline Neville-Jones, said, "If there is an increase in plots it doesn't follow I fear that the right way to deal with that is actually then to hold suspects for an even longer time.
"What you need if you've got an increase in plots is the right quantum of resources for both the police and the intelligence services to track and disrupt the plots and that's a question of bringing resources to bear."
And other opposition is heavyweight.
The Director of Public Prosecutions, Sir Ken McDonald, has made no secret of his opposition.
And the former attorney-general, Lord Goldsmith, repeated his objection to changing the law. He said the proposal "ultimately doesn't make sense".
Along with Shami Chakrabarti of the human rights organisation Liberty, he says introducing 42 days may alienate parts of the community, particularly Muslims, some of whom see the move as an attack on them.
When the issue comes to a parliamentary vote next month, the government could face a damaging defeat.
The Labour chair of the home affairs select committee, Keith Vaz, says ministers don't have enough support in parliament to get the measure through.
But David Winnick, one of the key rebel Labour MPs says the vote could go either way.
Nonetheless, a defeat on such a high profile issue would be very damaging for the prime minister - a serious dent in his already battered reputation.