Iran has released a second letter apparently written by the only female member of the 15-strong Royal Navy crew it captured in the Gulf.
The letter, apparently signed by Leading Seaman Faye Turney, says she is being treated "well and humanely".
It adds: "Isn't it time for us to start withdrawing our forces from Iraq and let them determine their own future?"
Chancellor Gordon Brown said using LS Turney for propaganda purposes was "cruel, callous and inhuman".
After the release of the second letter Foreign Secretary Margaret Beckett said: "We have not seen this letter but we have grave concerns about the circumstances in which it was prepared and issued."
On Wednesday Iranian TV showed footage of LS Turney, from Shrewsbury, in Shropshire, giving an interview in which she said the British crew had "trespassed" in Iranian waters - something the UK denies.
It was accompanied by another letter "written" by the 26-year-old sailor to her family.
UK VERSION OF EVENTS
1 Crew boards merchant ship 1.7NM inside Iraqi waters
2 HMS Cornwall was south-east of this, and inside Iraqi waters
3 Iran tells UK that merchant ship was at a different point, still within Iraqi waters
4 After UK points this out, Iran provides alternative position, now within Iranian waters
On Thursday, Ali Larijani, Iran's chief nuclear negotiator, said the release of LS Turney, which had been widely expected, had been delayed.
He talked about the UK's "incorrect attitude", which appeared to refer to the UK's refusal to agree the crew had been in Iranian waters.
At the UN headquarters in New York, the security council voiced its "grave concern" at Iran's capture of the 15 British sailors and called on Tehran to allow the UK consular access to them.
The statement is a watered-down version as the UK wanted it to "deplore" Iran's detention of the Britons and call for their immediate release.
Iran, which the UK later said had sent a note to the British embassy, responded by saying it was not helpful to try to engage third parties.
The UN secretary-general has spoken to Iran's foreign secretary about the issue, at an Arab summit in Saudi Arabia.
European Union foreign policy chief Javier Solana said he had also spoken briefly to Iran's foreign minister and reiterated the EU's call for an end to the "illegal" detention.
Separate to the letter, Iran also released on Thursday new footage of the British crew being seized which it said showed they were illegally in Iranian waters.
Iran's state TV also showed a military briefing in which charts were shown indicating the crew were in Iranian waters.
Before the release of the second letter Prime Minister Tony Blair said Iran's decision to show footage of LS Turney was a "disgrace".
"I just think it's completely wrong, a disgrace actually, when people are used in that way," he said.
"That's contrary to all international laws and conventions, and is not going to make any difference to us."
He added: "The important thing is we just keep making it very, very clear to the Iranian government it is not a situation that will be relieved by anything but the unconditional release of all our people."
Iran has not disclosed where the service personnel are being held, but the BBC has been told they are at a military base in Tehran.
Deputy Chief of the Defence Staff, Vice Admiral Charles Style, has produced data which he said proved the boarding party was 1.7 nautical miles inside Iraqi waters.
The Iranian embassy in London said the Britons had been 0.5 km inside Iranian territorial waters.