European Union states are considering recalling their ambassadors from Iran in an attempt to secure the release of the British embassy employees being held in Tehran.

EU diplomats said the envoys could be recalled temporarily in solidarity with locally engaged staff from the British mission in Tehran who have been accused of involvement in post-election rioting. The British government insists the accusations are false.

Nine people were arrested on Saturday. Five were freed today but four others understood to be the most senior were still being held and questioned. Gordon Brown said it was "unacceptable" and "unjustifiable" that the local employees were being held.

The prime minister was speaking at a press conference with the European commission president, José Manuel Barroso, who expressed his "full solidarity" with the UK. Yesterday, EU foreign ministers warned Iran that "harassment or intimidation" of embassy staff would be met with a "strong and collective" response. Most of the 27 EU member states have ambassadors in Tehran.

Meanwhile, Iranian officials have declared the official presidential election results, which saw the incumbent hardliner Mahmoud Ahmadinejad win by a landslide, to be correct after a partial recount.

"The secretary of the Guardian Council in a letter to the interior minister announced the final decision of the Council ... and declares the approval of the accuracy of the results of ... the presidential election," according to reports in the state TV broadcaster.

Requests for a new election and allegations of voting irregularities have been rejected.

Diplomats said it had not yet been agreed when the EU envoys would leave or for how long. But the news was clearly intended to signal seriousness of intent to Iranian government in the hope it would back down.

Iran's foreign ministry had earlier appeared to respond to the warning by saying it did not wish to damage or downgrade relations with the UK, after a telephone conversation today between David Miliband, the foreign secretary, and his Iranian counterpart, Manouchehr Mottaki.

The fear in London is that the Iranian foreign ministry is not in control of the situation, with hardliners from the interior ministry and intelligence service calling the shots as part of a campaign to pin the blame for the unrest on foreign governments.

Last week, as protests continued over the "stolen" election, Iran expelled two British diplomats – the embassy's second and third secretaries – in protest at their "undiplomatic" approach.

The two have arrived back in Britain, the foreign office confirmed, and the families of British embassy staff have also left Iran. In retaliation, Britain expelled two diplomats from the Iranian embassy in London.