This article is more than 7 years old

This article is more than 7 years old

The government will this week formally announce its plan to sell off Royal Mail in the most contentious privatisation since British Rail two decades ago.

Vince Cable, the business secretary, will make a statement to the House of Commons formally kickstarting the sale of the 497-year-old postal service.

Cable's statement – which is expected on Wednesday but could be delayed until next week – will set out details of the sale, expected to come via a £2-3bn flotation on the London Stock Exchange this autumn.

The statement will set out the terms under which 10% of the shares will be granted to postal workers. The public will also be encouraged to buy shares in an advertising blitz along the lines of the "Tell Sid" campaign that encouraged the £9bn sale of British Gas in 1986.

The formal announcement of the sale to MPs in parliament comes amid mounting outcry from postal workers who have taken to an open top bus to protest against the "great British flog-off".

Billy Hayes, general secretary of the CWU, which represents more than 100,000 postal workers, warned the government: "We aren't going to let Royal Mail be sold. We are going to fight every step of the way. A strike is certain."

Hayes warned the government that the union was already planning industrial action that could include rolling strikes this autumn and members would formally meet to vote on strike action on 31 July.

"The public don't want it [privatisation]. The workforce certainly don't want it," Hayes said as the union began a week-long open-top bus protest tour of parliament, Royal Mail's head office and the banks organising the sale.

Hayes said he hoped the threat of sustained industrial unrest would scare off institutional investors from buying shares in the state-owned delivery service and force the collapse of the sell-off.

His message to potential investors was: "Royal Mail is not for sale. Your shares are not going to be very safe. This is going to be a privatisation against a background of industrial unrest."

Hayes said the union's campaign had already resulted in several big investors deciding to steer clear of a privatised Royal Mail.

On Monday the union took its protest bus to the offices of the investment banks Goldman Sachs and UBS, which are leading a syndicate of banks that could collect about £30m from the privatisation.

"Goldman Sachs and UBS ruined the economy of this country and they are now working on the ruination of the Royal Mail, and we are not going to stand idly by and let that happen," Hayes said. "People have got more faith in postal workers than bankers at UBS and Goldman Sachs."

The government has also appointed Barclays, Bank of America Merrill Lynch, Investec, Nomura and Royal Bank of Canada to the banking syndicate working on the sale. The banks are set to collect 1% of the sale price, which is expected to be £2-3bn.

Via a loudspeaker on the brightly coloured protest bus John Simkins, a CWU London rep, shouted to passersby: "Reject the City fat cats. Protect your local postmen and women."

"Whose mail? Our mail," he chanted to ripples of applause and postal workers loudly playing vuvuzelas. "Don't let a few elite in the City make a fast buck at the expense of your Royal Mail. Once the City gets its hands on it you'll be lucky to get a delivery once a week."

The government has pledged that privatisation will not affect Royal Mail's universal service obligation to deliver to every address in Britain six days a week for the same price.

The government had attempted to win over workers by offering them an 8.6% pay rise, but the union rejected the "misleading and unacceptable" three-year offer.

The CWU said the pay rise deal was "only there as a sweetener to swallow some very bitter pills" including privatisation and "damaging" changes to pensions and working conditions.

Royal Mail's chief executive, Moya Greene, said the pay offer was a "good deal – good for Royal Mail and good for our people".

Royal Mail's annual report, which could be published this week, is expected to reveal that Greene collected a £500,000 increase in pay to £1.6m last year.

The average full-time postal worker's salary is £19,416. This would rise to £21,092 in 2015 if the union were to accept the pay deal, which includes a £300 lump sum in December. The company has already agreed to a minimum £100-a-year bonus.