Gordon Brown says increasing pre-charge detention for terror suspects to 42 days is the "right way to protect national security".
There are now only nine days to go until the crucial Commons vote on the issue, when the government faces the risk of another setback on an issue it has already been defeated on.
The article follows a letter Mr Brown received from Europe's human rights commissioner, Thomas Hammarberg, telling the prime minister the policy would put Britain at odds with Europe.
"I am concerned by the British government's suggestion to allow terrorism suspects to be detained for 42 days without charge," Mr Hammarberg writes.
"This would be way out of line with equivalent detention limits elsewhere in Europe.
"We need to be more restrictive with such measures. Keeping people detained for such long periods before prosecution is excessive and will prove counter-productive," he concluded.
At present the maximum limit on the amount of time terror suspects can be detained without charge is 28 days, longer than anywhere else in Europe. Civil liberties activists have argued even this is excessive.
The prime minister believes the "nature of modern terrorism and the growing complexity of investigations" means this is no longer enough, however. Despite widespread opposition he appears unwilling to back down.
Writing in the Times, he says: "The challenge for every government is to respond to the changing demands of national security, while upholding something that is at the heart of the British constitutional settlement: the preservation of civil liberties.
"And if the national interest requires new measures to safeguard our security, it is, in my view, the British way to make those changes in a manner that maximises the protection of individuals against arbitrary treatment."
The prime minister lays out the five "principles" the government is sticking to on the issue in the article. There will always be a maximum limit on detention and it will only be extended beyond 28 days in "truly exceptional circumstances".
Parliamentary approval, judicial oversight and independent reporting to parliament are the other measures Mr Brown believes constitutes "practical safeguards".
"With these protections in place, I believe parliament should take the right decision for national security," he adds.
"Having considered carefully all the evidence and arguments, I believe that, with all these protections against arbitrary treatment in place, allowing up to 42 days' pre-charge detention in these exceptional terrorist cases is the right way to protect national security."