The government's own human rights watchdog threatened last night to launch a legal challenge to Labour's plan to introduce a law that would let police detain terror suspects without charge for 42 days.

The Equality and Human Rights Commission says the key part of the counter-terrorism bill goes against human rights law and may breach the Race Relations Act.

As the home secretary, Jacqui Smith, renewed her appeal to Labour backbenchers to support the measure - amid growing international criticism - the EHRC prepared to brief MPs before the bill's second reading in the Commons tomorrow. The commission makes clear it will mount a legal challenge if the 42-day limit wins parliamentary backing.

"If adopted, we may seek to use our legal powers to challenge the lawfulness of the provisions and to establish clear legal principles on the use of pre-trial detention," it says in a briefing note to MPs.

The threat of a legal challenge from the EHRC, which has powers to take judicial review on legislation it considers may be in breach of the European Convention on Human Rights, is another setback to a government determined to increase the time terrorism suspects can be held without charge from 28 to 42 days.

Archbishop Desmond Tutu, Noam Chomsky and the American Civil Liberties Union have led an international outcry against the plan, which is opposed at home by the Tories and Liberal Democrats.

The government receives a further blow today when Lord Dear, the former chief inspector of constabulary, says a change in law would be a "propaganda coup" for al-Qaida. In a Guardian article, Dear writes that every chief constable he has spoken to regards the change as unnecessary.

Dear writes: "Make no mistake, extending pre-charge detention would most certainly be a propaganda coup for al-Qaida and its ilk. When I was an undergraduate reading law at university in the 60s, every self-respecting student had a poster of Che Guevara on their wall and knew something of the writings of [Herbert] Marcuse. Both of those terrorist luminaries said repeatedly that the best course for a terrorist was to provoke a government to overreact to a threat by eroding civil liberties, increasing executive powers and diminishing due process by the denial of justice."

The deep unease about the new measures was underlined by the EHRC. Set up last October under the chairman Trevor Phillips, it has specific power to take legal action over potential breaches of the Race Relations Act. The commission says it accepts that circumstances might arise which make an increase in the 28 day limit on pre-charge detention helpful to the police in obtaining evidence but this should not be at the expense of fundamental human rights.

It has told the Home Office that a "positive and compelling case" must be made before the maximum limit on pre-charge detention is raised, given its potential impact on liberty, the likelihood of its disproportionate impact on the Muslim community, and the risk of operational error.

"We consider that despite being restricted to particular and specific contingencies, the provisions set out by the Home Office are unlikely to meet threshold tests of public interest, justification or fairness," the commission adds.

It says the proposed safeguard of parliamentary scrutiny of each order will be meaningless without giving MPs detailed information on each suspect. Yet that would raise constitutional issues.

The commission says the proposed change also raises "very difficult issues" under race equality laws as it is being established to deal with a particular religious and racial minority. The EHRC believes it carries a high risk of damaging community relations, as Muslims are more likely to be regarded with suspicion.

The government is expected to win a Commons majority tomorrow for the bill's second reading. But Smith faces the prospect of defeat when detailed votes are held on the 42 days issue at the report stage in May after the local elections.

She said yesterday she believed the 42-day extension would be passed. But she told the BBC: "I hope parliamentarians will take their responsibility seriously to give those that we task with keeping us safe from terrorism the tools that we need to do it. I need to make the argument to parliament. As home secretary my responsibility is to do what I believe ... it is necessary to protect this country from the serious, sustained, and in some ways growing threat from international terrorism."

Smith stressed that the government was only taking a precautionary "reserve power" to increase the maximum period for detention without charge to be used in the exceptional circumstances of the country facing multiple terror plots.