The postal workers' union has urged the government to withhold any more payments to banks which advised it on its privatisation of the Royal Mail amid continuing complaints that the taxpayer has lost hundreds of millions of pounds over the sell-off.

The sale will come under the spotlight again when MPs question bankers who provided valuations of the company before the flotation.

The Communication Workers Union (CWU) said any further payments should be withheld on the basis that banks had undervalued the company.

The CWU's general secretary, Billy Hayes, said: "Hundreds of millions of pounds of taxpayers' money have been lost because of the failure of the government and its advisers to accurately value the company. In other situations this would be gross incompetence or even theft. Private shareholders have lined their pockets at the expense of the taxpayer following the huge leap in the share price.

"At the very least the institutions which advised the government should not receive any further payments, which are discretionary. Serious consideration should be given to claiming back fees paid for shoddy advice which has left the client – the taxpayer – out of pocket.

"It's time to see whose side the government is really on. Will they back their buddies in the city or stand up for the taxpayer and attempt to salvage some value for money from this debacle?"

Bosses from six banks will appear before the business select committee in parliament on Wednesday as controversy over the sale continues.

Those due to appear are John Mayne, managing director, UK client coverage, JP Morgan; Ben Storey, head of UK investment banking and broking, Citibank; Gert Zonneveld, managing director, co-head of research, Panmure Gordon; James Agnew, chairman of UK corporate broking, Deutsche Bank; James Robertson, managing director, UBS and Richard Cormack, managing director, co-head of equity capital markets, Goldman Sachs.

The business secretary, Vince Cable, and business minister, Michael Fallon, will appear before the committee next week, as well as a representative from Lazard, the institution that vetted the flotation price.

The committee said it wanted to follow up on issues that have arisen following the share flotation.

Goldman Sachs and UBS were the investment banks that ran Royal Mail's flotation last month; JP Morgan, Citibank and Deutsche Bank missed out on running the share sale; while broker Panmure Gordon attacked what it said was the undervaluation of Royal Mail.

The share price was set at 330p, valuing Royal Mail at £3.3bn, but the shares jumped by more than a third on the first day of trading, and broke through the 500p mark within a week.

The share price on Tuesday stood at more than 550p. The CWU said this meant the government had undervalued the company by more than £2bn.