Plans to extend the limit to 90 days were defeated in 2005
A Home Office minister says he believes plans to extend detention of terror suspects will get through Parliament - saying he thinks MPs "will buy it".
Tony McNulty told the BBC that concerns about the plans to extend the limit on holding suspects without charge from 28 to 42 days were "reasonable".
But the measures were not about locking people up "and throwing away the key", Mr McNulty told BBC Radio 4.
"Once people understand...the temporary nature then people do buy it," he said.
Mr McNulty was speaking amid mounting speculation that Gordon Brown could face his first serious Commons defeat as prime minister on the issue.
The Conservatives and Liberal Democrats oppose the extension of the limit, and a number of Labour MPs have indicated they are unhappy with the plans.
Keith Vaz, Labour chairman of the Commons home affairs committee, said he did not think ministers had enough support in parliament to carry the plans.
But Mr McNulty told BBC Radio 4's Today programme: "Recently myself and the home secretary have been speaking increasingly to people who have got some concerns about it and those concerns are reasonable.
"This is a very, very serious thing and I think once people understand the bulk of the model and the temporary nature then people do buy it."
Ministers have already made a number of concessions in an effort to win over potential rebel Labour MPs.
These include allowing Parliament a regular vote on whether to axe the powers and giving MPs the right to debate each occasion a suspect is held for more than 28 days.
"I would say to people quite candidly look at the model, look at what we are trying to do and I think it is a proportionate response to the situation," Mr McNulty said.
On Sunday, Home Secretary Jacqui Smith stressed the scale of the terrorist threat Britain faces, telling the News of the World 30 terror plots were being investigated.
Ms Smith said: "We now face a threat level that is severe. It's not getting any less, it's actually growing.
"There are 2,000 individuals they are monitoring. There are 200 networks. There are 30 active plots.
"We can't wait for an attack to succeed and then rush in new powers. We've got to stay ahead."
She said that the growing complexity of terror plots, and the international dimension of them, means there will come a time when investigators need more than 28 days before bringing charges.
Critics insist existing civil emergency laws could be invoked to extend the detention period if the situation became sufficiently serious.
The Counter Terrorism Bill passed its first Parliamentary hurdle earlier this month when it was given an unopposed second reading but it now goes to committee stage where MPs will scrutinise the detail.
Under the new proposals, the home secretary would be able to immediately extend the detention limit of a suspect from 28 to 42 days, as long as it was supported by a joint report by a chief constable and the director of public prosecutions.
The extension would then have to be approved by the Commons and the Lords within 30 days. But if either House voted against it, the power would end at midnight on the day of the debate.
Conservative security spokeswoman, Dame Pauline Neville-Jones, said: "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."
Lib Dem home affairs spokesman Chris Huhne said: "We have a 92% conviction rate on terrorist offences. For heavens sake, that's far higher than on ordinary criminal cases.
"So what on earth is the government on about in saying that we need to go further than any comparable country has anywhere else in the world?"