The head of the BBC on Friday accused Iran of intimidating staff members of its Persian service through slander, snooping and the arrest of their relatives.
BBC director-general Mark Thompson said "a campaign of bullying and harassment" against the broadcaster had worsened in recent months.
Thompson said in a blog post that last week the sister of a BBC Persian staff member was detained and held in solitary confinement in a Tehran prison. She has since been released.
He said it was one of several occasions on which employees' relatives have been detained and "urged to get their relatives in London to either stop working for the BBC, or to 'cooperate' with Iranian intelligence officials."
Thompson said staff members' email and Facebook accounts had been hacked, and they had faced false accusations of sexual assault, drug trafficking and financial crimes in official Iranian media outlets.
Britain's Middle East minister, Alistair Burt, said Iranian officials' "deplorable tactics illustrate again the deteriorating human rights situation in Iran, and the desperation of the Iranian regime to silence any independent voices."
"The international community has repeatedly called on the Iranian authorities to cease harassment and intimidation of journalists and to prevent illegal jamming of broadcasts," he said. "We will continue to do so."
Diplomatic relations between Britain and Iran have deteriorated as tensions mount over Tehran's disputed nuclear program.
Britain downgraded ties with Iran following a major attack on its embassy in Tehran in November, which it insists was sanctioned by the Islamic republic's ruling elite.
After the attack, Britain pulled all of its diplomats out of Iran and expelled Iranian diplomats from U.K. soil.
Iran has repeatedly jammed the broadcasts of BBC's Persian-language service and other international broadcasters, and last month Britain's broadcast regulator revoked the license of state-owned Iranian channel Press TV. It insisted the move was not politically motivated.
New York-based Human Rights Watch said Iran's harassment was part of a wave of arrests against journalists and bloggers ahead of parliamentary elections scheduled for next month.
"Detaining a BBC reporter's relative seems to be part of a wider campaign to harass Iranian journalists by putting pressure on them and their families," said Sarah Leah Whitson, the group's Middle East director.
"The recent wave of arrests, especially against relatives of journalists working abroad, is a reprehensible escalation in the current campaign to stifle freedom of information in Iran."