Foreign Secretary Jack Straw has urged Iran's new hardline president to address fears over the country's controversial nuclear programme.
Jack Straw wants reassurances over Iran's nuclear programme
The election of the ultra-conservative mayor of Tehran Mahmoud Ahmadinejad has raised concerns that the Islamic state might harden its nuclear stance.
Mr Ahmadinejad has already said his victory is a blow to Iran's enemies.
Lib Dem Sir Menzies Campbell warned of Middle East instability if Iran dodged its nuclear obligations.
"Any attempt to evade their responsibilities under the nuclear non-proliferation treaty could lead to great instability in the Middle East and concerted action against Iran through the United Nations," he said.
Shadow Foreign Secretary Dr Liam Fox said "economic reform and a halt to Iran's nuclear programme" were key to the security of Iran and the rest of the Middle East.
In a statement, the foriegn secretary said: "I hope that under Mr Ahmadinejad's presidency, Iran will take early steps to address international concerns about its nuclear programme and policies towards terrorism, human rights and the Middle East Peace Process.
He also warned of "serious deficiencies" in the way the election was run, echoing US claims that the poll was flawed.
"Candidates, senior figures in the regime and election officials have complained of widespread interference and electoral malpractice by the security forces and other regime organs in the first round."
Mr Ahmadinejad won 62% of the vote in Friday's run off poll - nearly twice that secured by his more moderate rival, former president Hashemi Rafsanjani.
Iran has said its nuclear programme is only intended to produce energy, but the US contends it is designed to develop a nuclear weapon.