THOUSANDS of Royal Mail ­investors have sought to cash in their shares, which almost doubled in value on their first full day of trading.

The official sharedealing services buckled under the strain of demand amid the huge volume of sales. Investors who received shares from the government had major problems selling through the ­official Equiniti phone sharedealing service.

Meanwhile, many other shareholders said they had yet to receive a reference number which is required to sell shares online.

Royal Mail shares soared in value to new heights on their first full day of stock market trading as their value soared from the original sale price of 330p to peak at 490.7p today.

Private investors who applied for between £750 and £10,000 worth of shares received 227 shares, initially worth £749.10 at the 330p float price. Yesterday those shareholders stood to get back £1113.89 if they managed to sell at the peak price - a £364.79 increase.

However, some shareholders complained they were unable to share their shares because they were still waiting for a key account number. Other investors discovered emails with the numbers were ending up as email spam.

One said: "Equiniti obviously did not expect a big demand on their phone lines today, as I did not get through until 1.30pm having tried all (morning)." Another investor said: "Took me 400 attempts to get through to #equiniti Royal Mail sale line. Price dropped 12p in the one to one and a half hours it took."

A spokesman for Equiniti said the company's phone lines had been swamped and apologised. He said the company was urgently trying to fix the problem.

The government warned that it could take up to 48 hours for emails detailing share allocations to be sent to all of the 690,000 investors who bought 227 shares in the flotation. The emails are required for online selling.

Business Secretary Vince Cable and investment bank Lazards are facing fresh questions from MPs over the privatisation amid concerns that ministers heavily underpriced the shares at 330p.

Mr Cable dismissed the steep rise when conditional trading began last week as "froth".

Its closing price of 489p implied a rise in the company's value of £140 million in one day, to reach £4.9 billion. The initial offer had valued Royal Mail at £3.3 billion.

Questions about the initial share price prompted Royal Mail chief executive Moya Greene to take the unusual step of agreeing with Mr Cable's comments - in effect implying her own company was overvalued in the market.

But Dave Ward, general ­secretary of the Communication Workers Union, said the soaring price was "more proof that the company was undervalued by the Government's City mates".

The union is today to announce a ballot result for strike action over issues such as pay and pensions linked to privatisation. Royal Mail chief executive Moya Greene described the first full day of trading as a "pivotal moment."