Commons business committee plans to write to large investors as part of bid to find out if state-owned postal service was undervalued

Royal Mail investors who bought large stakes in the postal service following its £3.3bn privatisation last month are to be asked by MPs why they have staked hundreds of millions of pounds on the view that the government sold the firm on the cheap.

The news emerged after the Commons business committee investigating the Royal Mail flotation questioned the business secretary, Vince Cable, and his ministerial colleague Michael Fallon how the offer was valued, prompting an assertion from Cable that there was no need for an independent inquiry into the process.

Committee chairman Adrian Bailey said he will be writing to the The Children's Investment Fund (TCI) and GIC, Singapore's sovereign wealth fund, which have built up their Royal Mail stakes since its listing to more than 6% and 4%, respectively – having decided the shares would rise far above their 330p flotation price.

He said: "Yes, we might well want to [write to major new shareholders to ask why they value Royal Mail so highly]. We are reviewing the transcript [of evidence] to identify areas to follow up."

The committee has been investigating whether the taxpayer has been shortchanged by the Royal Mail flotation, in which 60% of the shares were sold to outside investors last month. The share price has since soared by about 70%, prompting criticisms that the government could have demanded a higher price. The Bow Group, a thinktank led by former prime minister Sir John Major, has called for an independent inquiry into the privatisation.

When asked if he thought an inquiry was required, Cable replied: "Absolutely not. We think this is a good process for the taxpayer."

He added that the valuation was only one criteria in deciding whether or not the taxpayer had received value for money, as the company could have withered – and its services put at risk – without access to private capital to invest in its future.

"Bearing in mind the set of objectives which we set at the very beginning ... the value for money is partly dependent on the offer price, it's partly dependent on the continuing value of the state's [30%] share, and it's partly dependent on what happens to the company. If the company isn't able to invest successfully [in its business], you could be left with a serious casualty. When we take all those things together, I think the conclusion will be, when people have settled down, that this has been a very professional well-managed and successful operation."

Royal Mail floated at 330p a share when the government sold 600m shares last month. Once the shares began trading on the stock exchange, they quickly soared. The shares were up 5% on Wednesday afternoon following the group's first results statement as a public company, changing hands at around 563p.

Also being questioned alongside Cable and Fallon were Mark Russell, the chief executive of Shareholder Executive which holds state stakes in businesses, and William Rucker, the chief executive of the government's main financial adviser, Lazard.

Russell said the government had been taken by surprise by the surge in the share price, telling the committee: "We did not anticipate the share price to move to the extent that it did."

He added, however, it had been anticipated that the shares would rise following privatisation, which was part of the reason why the government had retained a 30% Royal Mail stake. Typically, the City hopes the shares rise by around 10% on the first few days of trading following a flotation.

Bailey also asked the witnesses if it was predictable that Royal Mail shares would surge so strongly, with the offer was 20 times oversubscribed by investors.

Lazard's Rucker claimed not: "A lot of the orders [for shares] that go into the books ... there is a heavy element of gaming. The three biggest orders were $1bn each. That would have represented 20% of the company. Those institutions had no expectations of ever receiving anything like that quantity of the stock."