Gordon Brown will push for an extension to the time terror suspects can be held without charge in a bill to be included in next month's Queen's Speech.
New safeguards to protect civil liberties have been stressed
But Downing Street is playing down reports he wants to double the current time limit to 56 days.
The prime minister wants to head off a potential backbench rebellion by stressing new safeguards to protect civil liberties.
He has promised greater judicial and Parliamentary oversight of detention.
The 28-day limit came into effect in July 2006 after rebel MPs defeated plans for 90-day detention.
The government has said that there have been no cases since then where a suspect has been released when a higher time limit would have led to a charge.
But it argues that there may be cases in the future where more than 28 days will be needed for charges to be brought.
And it is hoping that by stressing new safeguards it will be able to quell any backbench rebellion on increasing the upper time limit.
In a July statement to the Commons, Mr Brown suggested the current 28 day limit could be doubled.
He told MPs: "We are also proposing for consultation - and this would not require a state of emergency - an extension of the current limit for up to 28 days more or a lesser period."
In a speech on Thursday, Gordon Brown said: "In future 28 days may not be enough and we are also considering other proposals including post-charge questioning."
But he again stressed the need for greater oversight, adding: "There will be - and must be - greater protection for the individual, both greater legal or judicial safeguards on executive decisions and more intensive scrutiny of them by Parliament".
Downing Street earlier distanced itself from newspaper reports that Mr Brown would now push for 56 days.
An aide told BBC News no decision had been made on an upper time limit.
The government has said it wants cross-party agreement on anti-terror laws.
But the Conservatives and Lib Dems both insist they will not back any extension of detention until the government can provide evidence it is needed.
Shadow Home Secretary, David Davis, said: "We have always said that if there was evidence to support this we would look at it carefully.
"So far, not only have we not had a shred of evidence to support this, but we have had an admission from the home secretary that there is not one new iota of evidence to support it and that any proposal to extend the term is because 'they can imagine circumstances under which it would be necessary'.
"Gordon Brown has promised to 'write the next chapter in British liberty'.
"It would be a tragedy if this chapter proved to be an ill-thought through, politically motivated, curbing of the liberties that thousands, if not millions, of British citizens have died to defend."