A UK hedge fund has become the biggest shareholder of Royal Mail after the British government.
The Children's Investment Fund (TCI) has bought a 5.8% stake in Royal Mail worth £286m ($462m).
TCI is considered to be an activist investor which means it pressures management to make changes.
The Labour Party says TCI's investment is a sign that the privatisation is benefiting "big money investors in the City" and not the taxpayer.
Royal Mail shares were priced at 330p earlier this month, but have since risen by more than 50%, which has spurred critics who accuse the government of selling off Royal Mail too cheaply.
Ian Murray, Labour's Shadow Minister for Trade and Investment said: "David Cameron's Royal Mail fire sale has seen the vast majority of shares going to big money investors in the City while the taxpayer is left short changed."
Labour Peer Lord Sugar, perhaps best known for his role in the BBC1 series The Apprentice, become the latest critical voice, when he questioned government business spokesman Lord Popat in parliament.
He questioned the advice of UBS, Lazards and Goldman Sachs who advised the government over the price and timing of the sale.
"Bearing in mind other reputable banks had come on record giving a valuation of £5bn, why were these banks ignored and what will you be doing by way of an inquiry in finding out who the lucky institutions were that underwrote this bargain basement sale?" Lord Sugar said.
Lord Popat said that the government had been focused on securing "value for the taxpayer".
He also said that the banks who helped with the sale of the Royal Mail could receive up to £18.4m in fees.