The Government has been warned that it could face a legal challenge if it presses ahead with plans to extend the time terrorist suspects can be held without charge to 42 days.
The Equalities and Human Rights Commission said that it believed the plans set out in the Counter Terrorism Bill were discriminatory and in breach of the Magna Carta.
The commission's legal group director John Wadham said that the proposals were clearly aimed at the Muslim community while the process for determining whether a suspect was detained for an extended period was "flawed".
"This proposal in exceptional circumstances to have a 42 day detention period is in fact targeted at one community," he told the BBC Radio 4 Today programme.
"It doesn't say that in the Bill, but everyone knows that it is directed at one community. I imagine that 100% of the people who are arrested and detained will be from the Muslim community."
He said that six weeks was a long time to hold someone who may never even face charges.
"The process for making decisions about whether someone is or isn't going to be detained for long periods is flawed in itself and there are very significant issues as to how people will be treated," he said.
"Six weeks is a long time. It will result in problems for people's families, their homes, their jobs, and, of course, their mental health."
The warning came ahead of the first test of Commons opinion on Tuesday when MPs vote on whether to give the Bill a Second Reading.
Home Secretary Jacqui Smith said at the weekend that she had been receiving a "good response" in discussions with Labour MPs about the Bill, although many are thought to be deeply unhappy at the plans.
