Royal Mail has warned that more postal workers will lose their jobs following its controversial privatisation. On Tuesday the 500-year-old national institution saw unprecedented demand for its share offer, with more than 1 million people thought to have applied. In its written submission to parliament on Wednesday, Royal Mail said: "The company will employ fewer people in the future, whoever owns it."

The company refused to state how many of Royal Mail's 150,000 employees will be axed.

Moya Greene, Royal Mail's chief executive, who was paid £1.6m last year, has said the company needs to be "sized appropriately for the [declining] traffic we have to process".

The forthcoming cuts come on top of 50,000 jobs lost over the past decade. The company said it "remains committed to the overarching objective of achieving this without compulsory redundancies".

"Over the past decade, the postal services sector has changed dramatically. A decline in mail volumes has coincided with the liberalisation of the market and the emergence of competition. In recent years, we have seen a significant increase in the number of parcels being sent. All of this has meant a difficult process of change for our people," Royal Mail said in its submission to the business, innovation and skills select committee.

"Many of Royal Mail's employees have seen changes to their working practices as the company has adapted its operations to the changed mix of mail. Change will continue and the company will employ fewer people in the future, whoever owns it."

Royal Mail workers, who are 96% opposed to the privatisation, continued their campaign against the sell-off with a protest outside parliament ahead of committee meeting. Activists dressed up as highway robbers carried banners saying: "The Great British Royal Mail Robbery".

Vince Cable, the business secretary in charge of the sell-off, was giving evidence to MPs on Wednesday. Ahead of the hearing, the government said its main objective was to secure the universal postal service, which guarantees deliveries to every address in the UK, six days a week, for the same price: "Privatisation will allow the company to innovate, invest, improve our services, win new business, deliver the high-quality universal service, and continue to be a very substantial employer in the UK."

The government has been overwhelmed with public orders for shares in Royal Mail, up to 70% of which will be floated on the stock exchange on Friday morning. The shares are expected to rise instantly, by a speculated 20-40%.

The government will collect about £2bn from the sell-off, but Labour claims the coalition massively undervalued the company and could have made hundreds of millions more for taxpayers.