Iran twisted the knife in the hostage crisis today, releasing a letter said to be from the captured British sailor Faye Turney in which she called for British troops to be withdrawn from Iraq.
The letter, in which Tehran appeared to be using the hostages to try to dictate British foreign policy, was apparently designed to heap humiliation on British Prime Minister Tony Blair.
Its release came minutes after Mr Blair had said that he would not be drawn into any "tit for tat" deal to secure the release of the 15 servicemen, and took the stand-off into increasingly unpredictable territory.
A livid Mr Blair denounced the new letter, the second to be released by the Britons' captors.
"It is cruel and callous to do this to somebody in this position and playing this kind of game - it is a disgrace," said the Prime Minister's spokesman.
According to a text of the letter, released by the Iranian Embassy in London, the 26-year-old mother wrote: "Isn't it time for us to start withdrawing our forces from Iraq and let them determine their own future?"
Iran had earlier reneged on its promise to release Leading Seaman Turney and gone back on a pledge to let British diplomats visit the detainees, who have been held in secret locations since Friday.
As the standoff deteriorated, Iran threatened to put the 15 on trial and claimed it had satellite evidence that they breached its borders six times.
Today, however, Iran again dangled the possibility that Leading Seaman Turney might be freed soon.
Asked by Recep Tayyip Erdogan, the Turkish Prime Minister, to free the only woman among the British captives, Iranian President Mahmoud Ahmadinejad promised to study the request "in a positive manner", according to Iranian television.
The UN Security Council, voicing "grave concern", meanwhile called on Iran to allow consular access to the detained British naval personnel and urged "an early resolution of this problem, including the release of the 15".
Britain originally asked for a tougher three-sentence statement to "deplore" the detention of the British personnel and "support calls" for their immediate release, but this was blocked by Russia and several other members.
The latest letter from Leading Seaman Turney, dated Tuesday, was addressed to "representatives of the House of Commons". It said: "Unfortunately during the course of our mission we entered into Iranian waters.
"Even through our wrongdoing they have still treated us well and humanely, which I am and always will be eternally grateful for. I ask the representatives of the House of Commons, after the Government have promised that this type of incident would not happen again, why have they let this occur, and why has the Government not been questioned over this?"
A written and video confession by Leading Seaman Turney had previously been shown on Iranian TV, increasing concerns that she was being pressurised into making propaganda statements.
Iranian TV last night imitated the British Ministry of Defence briefing that launched Mr Blair's offensive.
A Tehran military officer was shown in front of a map of the Gulf, holding up an electronic positioning device seized from the Britons and pointing at numerous spots where he claimed they had trespassed.
Iran appeared to be punishing Mr Blair for a diplomatic offensive built around claims that Iranians deliberately lied when saying the Royal Navy had violated their waters.
By James Bone (New York), Dominic Kennedy and Philip Webster