Efforts to secure the release of 15 Royal Navy personnel held by Iran will enter a "different phase" if diplomatic moves fail, Tony Blair has said.
Downing Street said the UK could end up releasing evidence proving the group had not ventured into Iranian waters.
Foreign Secretary Margaret Beckett has called for their "speedy return".
One of the 15, Faye Turney told the BBC last week: "Sometimes you may be called upon, and when you do you've just got to deal with it and get on with it".
Meanwhile, the family of Ms Turney, from Shrewsbury, Shropshire, has said this is a "very distressing time" for them.
The BBC has been told the group are being held at an Iranian Revolutionary Guards Corps base in Tehran.
Iran says they were trespassing in Iranian waters when they were seized on Friday - but the prime minister said the group were in Iraqi waters under a UN mandate.
Mr Blair said the most important thing was the welfare of the eight sailors and seven marines from HMS Cornwall and securing their release.
The prime minister's official spokesman said Mr Blair's remarks about a "different phase" did not refer to any extreme diplomatic action, such as expelling Iranian diplomats from Britain or military action.
"We have been clearly stating that we are utterly certain that the personnel were in Iraqi waters.
"We so far have not made explicit why we know that, because we don't want to escalate this."
READ MORE: http://news.bbc.co.uk/1/hi/uk/6498611.stm