Britain on Tuesday completed the privatisation of the Royal Mail, offloading its final stake in a sale whose proceeds will be used to cut the national debt.

The government sold a 13-percent stake to institutional investors at a price of 455 pence per share, raising £591 million ($907 million, 800 million euros), the Department of Business, Innovation and Skills said in a statement.

Another 1.0-percent stake will be gifted to eligible Royal Mail employees in Britain, taking their total holding in the company to 12 percent.

Britain launched plans to sell off the Royal Mail in 2013 in the biggest privatisation since Margaret Thatcher sold off former state-owned firms British Gas and British Telecom to the public in the 1980s.

In total, the government has raised £3.3 billion from the sale.

British finance minister George Osborne welcomed the news, describing it as a "milestone moment" in the postal group's history.

"Every penny will be used to pay down national debt," Osborne added in a tweet.

The government sold off more than half of Royal Mail in 2013 but has since been accused of selling the company too cheaply at a launch price of 330 pence.

Another 15-percent stake was sold earlier this year at 500 pence per share.

However, the Communication Workers Union slammed the Conservative administration for the latest sale.

"This fire sale nails the lie that the Tories stand up for the interests of ordinary people," said CWU general secretary Dave Ward.

"The remaining government share in this profitable company should have been used to safeguard the public's voice in Royal Mail and ensure the continuation of daily deliveries to every address in the country."