News agencies say their flight is due to land in London around noon local time. It is not known whether they will speak to the media upon their arrival.
Before leaving Tehran, several of the crew members spoke to Iranian state television and expressed thanks for their release:
Here is part of the interview with the only female detainee, Faye Turney:
Interviewer: "How [do] you feel that Mr. Ahmadinejad announced your freedom?"
Turney: "Well happy, just relieved, thankful to go home. The treatment here's been great, but it would be nice to get back, get home and see my family."
Interviewer: "What message do you have for the Iranian people?"
Turney: "Just 'thank you' for letting us go and we apologize for our actions, but many thanks for having it in your hearts to let us go free."
"To the Iranian people, I can understand why you were insulted by our apparent intrusion into your waters," another crew member, Felix Carmen, told state television.
Several of the crew had previously apologized for trespassing into Iranian territory. The question of whether the Britons had indeed violated Iran's sovereignty was at the heart of the two-week crisis over their detention.
Basic Issue Of Trespass Still Disputed
Britain continues to maintain the vessel was unlawfully seized in Iraqi waters while on a UN mandated mission to control smuggling in the Gulf.
London says a satellite-based position locator carried by the vessel proves there was no trespass.
But Tehran maintains the same locator showed the British naval personnel were in Iranian waters when they were arrested by a patrol vessel of Iran's Islamic Revolution Guard.
Iranian President Mahmud Ahmadinejad told a Tehran news conference on April 4 that he had pardoned the crew as a gift to the British people and to mark the birthday of Islam's Prophet Muhammad and Easter, the Christian holy day.
"I announce that the Iranian nation and the government of the Islamic republic -- while insisting on our power and right to try these military personnel -- have pardoned these 15 people, and we offer their freedom to the British people," he said.
International Reaction
London accepted the release with words that welcomed the end to the crisis and appeared intended to reduce the tensions it had caused.
"To the Iranian people I would simply say this: We bear you no ill will," British Prime Minister Tony Blair said. "On the contrary -- we respect Iran as an ancient civilization, as a nation with a proud and dignified history. And the disagreements that we have with your government, we wish to resolve peacefully, through dialogue."
Blair also thanked Britain's allies in Europe, in the UN Security Council, and in the Middle East for their support in helping to resolve the crisis.
In Washington, State Department spokesman Sean McCormack on April 4 reacted to the end to the crisis in similar terms.
McCormack said Washington welcomes the diplomatic success and that the United States bears no ill will toward Iranians.
Ahmadinejad's announcement came after Iran earlier welcomed a "change of tone" from London in the crisis.
The seizure of the naval personnel came during a perilous time in Iran's relations with the West, with the United States refusing to rule out military action over the Iranian nuclear program and the United Nations imposing tough new sanctions.
Behind The Scenes
Ahmadinejad's announcement followed reports in Iran's media that an Iranian diplomat would be permitted to meet with five fellow countrymen in Iraq, where they are being held in U.S. military custody.
But Bush said on April 3 in Washington that there would be no reciprocal release of the Iranian detainees in exchange for the release of the Britons. And today, the State Department said it was aware of no such connection.
In such situations, militarily strong Western nations such as Britain and the United States ordinarily bring a variety of diplomatic pressures to bear -- backed up with a remote, but real, threat of force. That's according to James Phillips, who studies international security issues at the Heritage Foundation, a Washington policy research center.
"The use of force is usually a last resort, but keeping it on the table strengthens diplomacy, which is the primary means of exerting pressure in cases like this," Phillips said. And, Britain and perhaps the U.S., but also Britain's other allies in NATO or the EU probably made it clear to Iran that this would have repercussions on a wide variety of diplomatic issues that would, in the long run, hurt Iran's national interest."
The Supreme Leader?
Phillips also cautioned that the release may have been prompted as much by internal pressures within Iran. He said Iran's supreme leader, Ali Khamenei, occasionally reminds Ahmadinejad that as president, he is only of secondary importance in the country's political hierarchy.
In this case, Phillips said, Khamenei may seen the crisis over the captured Britons leading to a diplomatic disaster for Iran, so he may have ordered Ahmadinejad to release them.
"If Khamenei says, 'Jump,' Ahmadinejad's going to jump," Phillips said. "He may resent it, but I think he did basically what he was told."
Phillips said he takes the U.S. and British governments at their word that there is no connection between the status of the Iranians held in Iraq and the release of the British service personnel.