Replying to criticism from UN High Commissioner for Human Rights Louise Arbour, the government said its Terrorism Bill -- drawn up partly in response to last July's London bombings -- was a "proportionate and necessary response" to the threat.
Arbour's comments, in a letter to the government dated November 28, could give ammunition to opponents of the proposals.
The bill, which would outlaw incitement to terrorism and extend the period suspects can be held without charge, is being scrutinised by the House of Lords after receiving a rough ride in the House of Commons.
Arbour said a number of clauses could violate Britain's obligations under international human rights treaties.
"I fear that certain provisions of this bill could pose grave challenges to effective human rights protection and set worrying precedents in the global struggle against terrorism," she wrote.
The terrorism legislation dealt Prime Minister Tony Blair his first major parliamentary defeat in November when MPs rejected government plans to extend the period over which police can hold terrorism suspects without charge to 90 days from 14.
The House of Commons voted to extend the period to 28 days.
But despite the shorter time, Arbour said she was "gravely concerned" about how human rights would be guaranteed.
She also raised concerns about the legality of clauses of the bill that would outlaw incitement to terrorism, the dissemination of terrorism-related publications and glorification of terrorism, among others.
Home Secretary Charles Clarke defended the measures.
"The High Commissioner has a number of specific concerns about the bill. We do not believe that these are justified," he said in a letter to Arbour published on Monday.
Clarke said he was satisfied the 28-day detention plan was compatible with the country's human rights obligations, particularly since it has safeguards including regular judicial oversight.
He also said a number of changes had been made to the bill in parliament and that more were planned.
"The government is satisfied that these clauses constitute a proportionate and necessary response to the threat of terrorism," Clarke said.
By Katherine Baldwin