Royal Mail's stock market flotation could leave the Kuwaiti royal family with greater control over the business than the Queen, unions have warned.

Billy Hayes, leader of the Communication Workers Union, said that the news that Royal Mail's shares soared by 36% at the start of trading on Friday would not be met with "celebrations in delivery offices throughout the country".

"We have got Sovereign wealth funds [investing]. Kuwait has bought shares in Royal Mail, Singapore has bought shares. We're going to have a situation where the royal family in Kuwait has more influence over the postal service in the UK than the royal family in this country."

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The Kuwait Investment Office (KIO) is reported to have been allocated around £50 million in Royal Mail shares, which would make it a major shareholder.

The KIO, the City-based wing of the Gulf state's sovereign fund, is understood to have bid for millions of pounds in shares.

The Queen's head has been on stamps during her reign, with Royal Mail always seeking the monarch's permission to use their image on its stamps.

Vince Cable said most of the shares had gone to "long term investors" like pension funds and insurance companies.

The Lib Dem business secretary dismissed concerns expressed about the Kuwaiti royal family's influence over the Royal Mail as "completely meaningless", saying that the Kuwait Investment Authority was "a long term investment institution which is very active in the UK".

The Royal Mail's stock market entry comes as CWU union members are set to vote on strike action over pensions and working conditions next week.

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