Newly-privatised Royal Mail will today face the prospect of a national strike by more than 100,000 postal workers amid continuing controversy over the "scramble" to sell off the company.
The Communication Workers Union (CWU) has balloted around 115,000 of its members for industrial action over issues linked to the sell-off, including pay and pensions.
Officials are confident of a yes vote when the result is announced later today, which would threaten disruption to mail deliveries in the run up to the busy Christmas period.
The union, which will have to give seven days notice of a walkout, has accused the Government of "deliberately creating a scramble" for shares, leading to many private investors cashing in their allocation at a big profit.
As formal trading started on the London Stock Exchange yesterday shares gained 15p, or 3%, making them almost 50% more valuable than the Government's price tag last week.
In the first day of dealing for many of the 690,000 small investors who bought stock in the highest-profile privatisation for years, shares were up to 490p to value Royal Mail at £4.9 billion.
That compares with the 330p per share price they were sold for by the Government on Thursday, which valued the group at £3.3 billion.
The share surge sparked further anger about public assets being sold too cheaply.
Dave Ward, CWU deputy general secretary, said: "The Royal Mail share price has soared further, bringing more proof that the company was undervalued by the Government's City mates.
"The taxpayer has lost over £1 billion already in this bungled fire sale of a cherished national institution. Postal workers cannot trade their shares for three years and they are far more concerned about their jobs than the share price.
"We're confident our members will return a yes vote in the ballot result for strike action, strengthening our position to secure a deal on protecting jobs, services and terms and conditions in the company.
"The share price increase is making profits for wealthy private investors and faceless institutions - it's not bringing any money into Royal Mail. The investment argument is a clear myth
"We will not accept people maximising individual profit on the back of minimising the value, terms and conditions of postal workers.
"We're determined this privatisation will not lead to the kind of job losses and downward pressure on pay and conditions we've seen in other industries and we're seeking a legally-binding agreement to protect jobs."
Ballot papers were sent to over 115,000 Royal Mail and Parcelforce workers in the first national strike ballot in the company since 2009.
The ballot is over pay, job security and terms and conditions - all of which the union maintains have been made more urgent following privatisation.
Meanwhile, Business Secretary Vince Cable has been asked to give fresh evidence to a committee of MPs over the privatisation following an appearance last week, when he faced tough questioning about the sell-off.
Investment bank Lazard is also set to be questioned next month by the Business Select Committee over the pricing of Royal Mail and concerns that institutional investors were allowed to buy into the company too cheaply.
Royal Mail said that around 100 million free shares have been allocated for eligible employees of the company, giving a total initial market value of approximately £489 million.
Each eligible full-time employee in Royal Mail is entitled to 725 free shares with an initial market value of £3,545.25.
Under HMRC rules, the maximum amount of free shares that can be allocated to an individual employee in any tax year is £3,000.
Royal Mail said due to the strong performance of the shares, this limit has been exceeded.
A total 613 shares worth £2,997.57 at the closing mid-price today have been awarded to each eligible full-time employee.
Less than 0.5% of around 150,000 of eligible full-time and part-time Royal Mail employees based in the UK chose to opt out of receiving free shares