Cracking down on bankers' bonuses is a "moment of truth" for the coalition government, a leading Liberal Democrat has warned, amid mounting expectations that payments of at least £7bn will be awarded in the coming weeks.

As campaigners for a "Robin Hood" transaction tax stepped up their campaign to raise £20bn from banks, the coalition was also under fire from opposition MPs for not doing more to end the bonus culture, which has been under scrutiny since the 2008 banking crisis. Lord Oakeshott, a Liberal Democrat treasury spokesman, said this was a moment for the coalition to act, particularly on forcing more disclosure from banks on the amount they pay.

"This is the coalition's moment of truth on fairness," he said. "The first item on our coalition agreement is a promise to deal with unacceptable bankers' bonuses. Secret bonuses are by definition unacceptable, so they must be disclosed."

While banks insisted no decisions had yet been taken on any payouts for the 2010 financial year, Bob Diamond, the new chief executive of Barclays, will find it difficult to dodge questions when he appears before the Treasury select committee on Tuesday. Diamond, who has not accepted a cash bonus for the past two years after taking home up to £75m over five years in performance-related share deals, will appear before the committee less than a fortnight after taking the helm of Barclays following a decade running its investment bank. Barclays is expected to pay £2.5bn in bonuses this year while bailed-out Royal Bank of Scotland is thought to be considering a £1bn bonus pool. The CEBR thinktank has estimated total City bonuses for 2010 at £7bn.

Len McCluskey, general secretary elect of Unite, said: "It is intolerable that within taxpayer funded institutions these undeserving rich bankers have not been forced to change their culture of greed and excess."

The government has backed down from Labour plans to force banks to reveal how many employees earn more than £500,000. Labour wanted banks to publish the details in bands beginning at £500,000 to £1m and increasing in £1m bands thereafter. Oakeshott said: "We now know 172 civil servants earned more than the prime minister, but thousands of public sector bankers at Lloyds and Royal Bank of Scotland earn far more. It is utterly unacceptable to hide what we pay them."

He called for action from UK Financial Investments, set up in the crisis to manage government stakes in bailed-out banks: "The Treasury abdicated responsibility for control of bonuses to UKFI under Labour and can't go on turning a blind eye."

Labour MPs heaped pressure on the chancellor, George Osborne. Chuka Umunna, who sits on the Treasury select committee, said: "In 2009 the chancellor said government should act in the light of unacceptable bank bonuses, stating that we could not wait for a 'promised land of a responsible bonus culture' but, now in power, he chooses to sit on his hands."Barely two years after being bailed out by the taxpayer, banks are trying to offer a pact to government to lend £200bn to small and big businesses but show some sort of restraint on bonuses in return for avoiding a tax on bonuses of the kind implemented by Alistair Darling a year ago.

A Treasury spokesman said: "The banks have put a number of proposals on the table. The government affirmed its desire to see a strong, responsibly and internationally competitive financial sector. The dialogue was constructive. These proposals will continue to be discussed."