The BBC has complained to the United Nations after it said the Iranian authorities launched an investigation into more than 150 of its staff for "conspiracy against national security."

The investigation follows a "sustained campaign of intimidation and harassment" by the Iranian government designed to pressure journalists to cease working for the BBC, the broadcaster said in a complaint to David Kaye, the UN special Rapporteur on the the promotion and protection of the right to freedom of expression.

Iranian authorities are investigating 152 Persian service staff, former staff, and contributors for conspiracy against national security, the BBC said in a statement.

The government has also imposed an asset freeze on BBC Persian staff, preventing them from inheriting family assets or from selling assets such as property or cars in Iran, it said.

“The Iranian government is conducting what appears to be a politically motivated investigation into 152 BBC Persian staff, former staff and contributors, accusing them of conspiracy against national security in I ran. This is an unprecedented collective punishment of journalists who are simply doing their jobs," said Tony Hall, director general of the BBC, in a statement.