LONDON (REUTERS) - Britain will push ahead with its biggest privatisation in decades when it unveils plans for a 2.5 billion pound ($$4.8 billion) stock listing of state postal operator Royal Mail this week, Sky News reported.

Business Minister Michael Fallon is expected to disclose the news in a statement to the House of Commons on Wednesday, Sky said on Monday, citing sources.

A spokesman for the Business Innovation and Skills department handling the sale said it would be providing lawmakers with an update by Thursday, July 18, before parliament begins its summer recess. It did not confirm the exact timing of the announcement or comment on its contents.

The decision to push ahead with privatisation will be a blow to trade unions, which argue the sale will spark a decline in postal service provision and working conditions for the company's 150,000 employees.

Royal Mail, whose history dates back to 1516 and King Henry VIII, and the government, argue access to external capital is vital to the firm if it is to continue shifting its business towards a future focused more on parcels.

Union members are touring London this week in a bus, taking their campaign to Goldman Sachs and UBS, the banks advising the government on the flotation.

As part of the government legislation, Royal Mail staff would have at least a 10 per cent stake in the group. Sky said the government was leaning towards giving the equity to staff for free rather than at a discount.

Sky also reported that Fallon would likely confirm expectations that members of the public would be able to participate in the London listing.

Royal Mail, which in May reported the company had more than doubled annual profit on the back of an online shopping boom, declined to comment.