Blair told journalists in Berlin that Iran's action in detaining the 15 sailors on March 23 was "unjustified and wrong."
"I hope the Iranian government understand how fundamental an issue this is for us," Blair told journalists. "We have certainly sent those messages back to them very, very clearly indeed. I hope that this can be resolved over the next few days, but the quicker it is resolved the easier it will be for all of us."
He said the sailors were definitely in Iraqi waters, and had not strayed into Iranian terrritory.
The Iranian Foreign Ministry earlier summoned Britain's envoy to Tehran to protest the "illegal entry" of the British sailors into Iranian waters. Iranian officials say the sailors have admitted entering Iranian waters.
Britain says the sailors were conducting routine antismuggling checks when they were seized in the Shatt Al-Arab waterway.