"It's plain for everybody to see that these were from the start flawed elections in which at least half the constituencies, reformist candidates were not on offer to the electorate," he told reporters on Monday.
"And by all accounts, the turnout is down by 25 percentage points from its level when there were free elections in 1997," Straw said as he arrived in Brussels for a meeting of EU foreign ministers.
The elections produced the expected large majority for supporters of the hardline Islamic regime, but many reformists had called for a voter boycott and hoped for a weak turnout to discredit the result.
Meanwhile, the conservative Guardians Council in Iran on Monday accused the reformist-dominated interior ministry of "playing with figures" to lower the politically sensitive turnout rate in the elections.
The council disputed the official 50.57% turnout rate given for Friday's polls, the lowest for a major election on the 25-year history of the Islamic republic, and claimed that the real figure was closer to 60%.
"It's plain for everybody to see that these were from the start flawed elections. In at least half the constituencies, reformist candidates were not on offer to the electorate."
British Foreign Secretary
The Guardians Council said in a statement on its website the interior ministry, controlled by pro-reform officials, "did everything possible to dissuade" Iranians from voting.
"Now they are resorting to playing with figures," said the council, a pillar of the clerical regime.
The council, an oversight body that had sparked the anger of reformists by disqualifying most of their candidates in advance, also took issue with the interior ministry's figure for eligible voters.
It said the ministry put the number of potential voters over the age of 15 at about 46.3 million for this year's poll, whereas the size electorate for last year's local election was 40.5 million.
"This would mean that in 1989, some 5,849,249 people were born, and none of those who were born in that year died," the council said.