His comments come after David Miliband, the British foreign secretary, insisted that there was no dispute between the countries over the arrests, which occured on November 25.
"There is certainly no confrontation or argument. As far as we are aware these people are being well-treated, which is right and what we'd expect from a country like Iran," he told the BBC.
Britain's foreign office has said it had some "limited indirect contact" with the five but could not say where they were being held or if they were in prison.
The incident comes amid heightened tensions between Tehran and the West over Iran's nuclear programme, and months after Iran accused Britain of involvement in its post-election violence.
Iran's Revolutionary Guards said on Tuesday it was their duty to confront "foreign forces" in the Islamic Republic's waters.
According to Iran's ISNA news agency, a group of students are expected to protest outside the British embassy in Tehran, the capital, over the sailors' "illegal entry" into their country's waters.
Shirin Ebadi, Iran's Nobel Peace laureate, has urged her government to free the sailors.
"Unfortunately, in this case, instead of warning them to leave the area, they arrested them. I believe that the Iranian government should immediately free them.
The detentions, announced by Britain on Monday, came after the sailors' yacht, The Kingdom of Bahrain, reportedly strayed into Iranian waters while travelling from Bahrain to Dubai.
Some British media have drawn parallels with a previous incident in 2007, when eight British navy sailors and seven marines were held for a month by Revolutionary Guards in a waterway that separates Iran and Iraq.
But Miliband stressed the two events were not related.
"It is important to say that these are civilians, not Royal Naval personnel," he said.
"They are yachtsmen, they were going about their sport and it seems they may have strayed inadvertently into Iranian waters."
Media reports have speculated that The Kingdom of Bahrain, owned by Sail Bahrain, may have drifted after breaking its propeller.
The yacht was detained on November 25, a day before it was due to take part in the Dubai-Muscat Offshore Sailing Race.
The crew have been named as Oliver Smith, Oliver Young, Sam Usher, Luke Porter and David Bloomer, according to informed sources in London.