Speaking as Tehran braced for new demonstrations, Prime Minister Gordon Brown urged authorities there to refrain from violence in response to peaceful protests.
"The elections are a matter for the Iranian people, but if there are serious questions that are now being asked about the conduct of the elections, they have got to be answered," he said.
"There must be no violence in response to peaceful protests," he added, after seven people were killed in demonstrations in Iran on Monday.
Foreign Secretary David Miliband insisted that the West is not taking sides over Iran, where elections on Friday saw Mahmoud Ahmadinejad win another term president, defeating reformist former premier Mir Hossein Mousavi.
"The long thesis of the conspiracy of foreign powers against Iran is one that is deeply ingrained in the popular imagination and peddled vociferously by the regime," he told the BBC.
But he said: "This is not a pro-West versus an anti-West competition in Iran, it is a competition to reflect the will of the Iranian people and I think that we have to hold fast to that point."
Brown meanwhile reiterated a warning that Iran's response to the demonstrations will determine its future relations with the West.
"I think Iran has got to listen very carefully," he said.
"The relationship they will have and the respect they will have from the rest of the world will depend on how they respond to what are legitimate grievances that are being expressed and have to be answered."
