Iran will take serious measures against five British yachtsmen detained in the Gulf if it proves they had "evil intentions", a close aide to Iranian President Mahmoud Ahmadinejad said yesterday.

Relations between Britain and Iran have been dogged by tension in recent years over a range of issues, from Tehran's nuclear programme to Iranian allegations of British involvement in post-election violence in June this year.

"The judiciary will decide about the five... naturally our measures will be hard and serious if we find out they had evil intentions," Esfandiar Rahim-Mashaie, the President's chief of staff, told the semi-official Fars news agency.

Britain stressed the five men were civilians and played down parallels with an incident in March 2007 when Iran seized eight British Royal Navy sailors and seven marines off its coast.

"There is certainly no question of any malicious intent on the part of these five young people," British Foreign Secretary David Miliband told BBC television.

Mr Miliband later told Parliament that he saw no political dimension to the case and that the men were being treated well.

"I think it is important to say there is no link at all between the position of the yachtsmen and the Iranian nuclear file or other political issues that exist between Iran and the international community," Mr Miliband said.