Jacqui Smith has come under fire for "muddying the waters" on detention
A row has broken out between ministers and civil liberties groups over how Britain compares with other countries on pre-charge detention times.
The Home Office says it is not possible to make a proper comparison because each country's legal system differs.
But the civil liberties groups say it is possible - and the UK is among the worst in the world for imprisoning people without charge.
It comes as ministers try to extend the pre-charge detention period to 42 days.
The dispute began last year when Liberty published a table which it said proved that the existing 28-day time limit in the UK for pre-charge detention "far exceeded" equivalent limits in other comparable democracies.
A comparison between the UK and 14 other countries showed Australia following the UK with a time limit of just 12 days. Turkey comes third with 7.5 days and Ireland fourth, with a one-week limit on holding suspects without charge.
In its report Liberty said: "Any extension to pre-charge detention would put the UK further out of line with comparable democracies around the world."
On Wednesday the Home Office issued a rebuttal in the form of a letter from the Home Secretary Jacqui Smith. In it, Ms Smith accused Liberty's international comparisons of being "misleading".
She said: "Because there is such variety between criminal justice systems in different countries... it is simply not the case of comparing like with like.
"Even organisations like Justice accept that such comparisons are wrong."
But when Justice - a respected human rights organisation made up of senior lawyers and judges - learnt of the letter, they immediately demanded a retraction.
And the group joined Liberty in insisting that the UK had the worst record amongst democratic countries for detaining people without charging them.
In a statement the group accused the home secretary of "wrongly claiming" that Justice disputed Liberty’s comparisons between periods of detention in the UK and other countries.
Justice director of human rights policy Eric Metcalfe said: "Unfortunately for the home secretary, the international comparisons are clear.
"The UK already has the longest period of pre-charge detention of any western democracy, and 42 days would push it even further ahead."
A Home Office spokeswoman said ministers would need to consider Justice's statement further before making a comment.
Liberty said the Home Office had also "muddied the waters" by claiming that other countries - such as France and Italy - had such different legal systems that comparisons about pre-charge detention times were invalid.
Liberty insisted its comparisons were valid, and said it had the support of respected experts both in the UK and abroad to back its research - including France's chief counter terrorism prosecutor, Anne Kostomaroff.
In a letter to Liberty Ms Kostomaroff stated: "Is the analysis of the French law in... Liberty’s attached report 'Terrorism Pre-Charge Detention: Comparative Law Study' accurate?
"Yes it is."
Jago Russell, who wrote the report for Liberty, said: "The conclusions of our study are no doubt embarrassing reading for a government insistent on pushing for 42 days detention.
"When so many other countries manage with pre-charge detention periods of less than a week how can the UK need to hold people for over a month?”
The Home Office said it would need to see Ms Kostomaroff's letter in full before making any statement.