"We are able to confirm that one of our staff remains in detention," a Foreign Office spokeswoman said, adding that the embassy worker was released last night.
"It remains our top priority to get all of our embassy staff released as soon as possible," the spokeswoman added.
The confirmation comes after Foreign Secretary David Miliband said Sunday that he expected the imminent release of the worker without charge.
Miliband said the international community was united in opposition towards "intimidation" from Tehran, which arrested a total of nine locally engaged embassy staff on June 28 in the wake of Iran's disupted presidential elections.
Seven were later released.
Iran accused the embassy employees of having instigated riots in the unrest that erupted over the re-election of President Mahmoud Ahmadinejad, which his rivals said was fraudulent and marred by widespread irregularities.
Iran's powerful Guardians Council said Friday that some of the British staff would be put on trial for their alleged role in stoking violence after the June 12 election, but did not say how many.
Tehran has also expelled two British diplomats from Iran, with Britain expelling two Iranian diplomats in a tit-for-tat measure.
Iran's supreme leader has described Britain, which has long had turbulent relations with Iran and a long history of mistrust, as the "most evil" of its enemies.
On Friday, EU member states summoned Iranian ambassadors in their capitals to protest at the detention of the Iranians working for the British embassy, who do not enjoy diplomatic immunity.
