Britain said on Sunday it had frozen Libyan leader Muammar Gaddafi's assets and lifted his diplomatic immunity, putting pressure on him to step down after his government's bloody crackdown on a revolt against his rule.

London moved quickly to implement United Nations' sanctions freezing the assets of Gaddafi, his daughter and four sons.

The Gaddafi family is reported to have billions of dollars of investments in London, while Gaddafi's son, Saif al-Islam, owns a GBP10 million ($16 million) home there.

Foreign Secretary William Hague said he had revoked the diplomatic immunity of Gaddafi and his family in Britain.

"We are now putting serious pressure on this regime," British Prime Minister David Cameron said in a televised statement. "There will be further export measures that will be taken as well in the coming hours.”

"It is time for Colonel Gaddafi to go and to go now. There is no future for Libya that includes him," he added.

Officials could not estimate the value of the Gaddafi family's assets in Britain, but said they could be substantial.

It is unclear whether the asset freeze will affect Libya's sovereign wealth fund, the Libyan Investment Authority (LIA), which is estimated to manage assets of around $70 billion, including stakes in European blue chips like British publishing group Pearson.

The freeze covers not only assets held in the names of Gaddafi and his children but assets controlled by them.

"The financial sector ... should bear in mind that Muammar Gaddafi and his family have considerable control over the Libyan state and its enterprises...," the Treasury said.

Banknotes

The British government also barred the export of uncirculated Libyan banknotes from Britain without a licence.

Removing the diplomatic immunity of Gaddafi and his family means they could not visit Britain without a visa, which would be denied under the UN's new travel ban.

In the event of any prosecution, Gaddafi would also lose protection from arrest in Britain. The UN Security Council called on Saturday for an International Criminal Court investigation of the Libyan crackdown.

British military transport planes evacuated 150 Britons and other foreign oil workers from desert camps in Libya on Sunday after flying out a similar number on Saturday.

The Ministry of Defence said one of the transport planes had suffered minor damage, apparently caused by small arms fire.

Hague said Gaddafi appeared to have stocks of mustard gas, a potentially deadly chemical weapon, that had not been destroyed under a 2003 agreement to dismantle weapons of mass destruction (WMD), though it was unclear what condition they were in.

Peter Mandelson, a close confidant of former British Prime Minister Tony Blair, told the BBC that Blair had been in contact with Gaddafi in the last few days.

For years, Britain treated Libya as a rogue state. But after the 2003 agreement on WMD, Blair helped lead Gaddafi back into the international fold, paving the way for big British business deals in Libya including by oil major BP.

In 2009, the Scottish government decided to release on compassionate grounds Abdel Basset al-Megrahi, the Libyan convicted of the 1988 Lockerbie airline bombing, which killed 270 people, including 189 Americans. That provoked a crisis in Britain's relations with the United States.

The coalition government says releasing Megrahi was wrong. – Reuters