The Prime Minister has announced that cash bonuses paid at the state-owned bank would be capped at £2000 for the third year in a row, but critics accused the Conservative leader of failing to crack down on bonuses paid in shares, which are expected to run into many millions of pounds.
A new European law requires banks to ask their shareholders to agree to payments worth more than 100% of salary.
Labour wants the UK Govern-ment, as the major shareholder in RBS, to reject any such payouts.
Under pressure from Labour leader Ed Miliband in the House of Commons, Mr Cameron refused to make any such commitment.
He did say that if there were attempts to increase "the overall bill, that is the pay and bonus bill ... we will veto it."
Labour sources attacked that pledge, pointing out that staff numbers at RBS have been cut significantly since 2008, with more redundancies planned.
An aide to Mr Miliband said: "The Prime Minister keeps referring to the total bonus pool but what is quite clear is the numbers employed in RBS have fallen quite dramatically; overall 40,000 across the whole of RBS since the financial crisis. In June last year, it announced it was cutting the number of people at its investment bank by 2000, that's over 20% in 18 months."
He added: "It was not long ago - 2009 - that George Osborne said 'it is totally unacceptable for bank bonuses to be paid on the back of taxpayer guarantees; it must stop'. What we did not hear today was an answer from the Prime Minister why he has changed that position. It's quite clear they are going to do nothing to rein in these excessive bonuses.
"It seems to us they have collapsed in the face of pressure from the bank RBS."
Downing Street said it could not comment specifically as it has yet to see any proposals on bonuses from Edinburgh-based RBS.
No 10 sources also warned that there could be unintended consequences of a clampdown. They said that in Europe there had been a move to increase salaries instead, which were harder to "claw back" should anything go wrong.
The Governor of the Bank of England, Mark Carney, said yesterday he did not back a "crude bonus cap" on pay in banks or proposals to restrict their size, expected to be announced by Mr Miliband in a speech on Friday.
Mr Carney told the Treasury Select Committee he "absolutely" agreed with the conclusions of the Parliamentary Commission on Banking Standards that it was unconvinced that was the right way to control pay. The governor also backed a commission led by Sir John Vickers that a cap on banks' market share would "not result in substantial improvement to competition".
Mr Carney said: "Just breaking up an institution doesn't necess-arily create or enable a more intensive competitive structure."
The UK Government is attempting to challenge the EU bonus cap which means pay-outs of more than 100% of basic salary must be approved by shareholders and Chancellor George Osborne warned the cap would not lead to bankers' receiving less money, just higher basic salaries.
He said: "What they will lead to is a Fred Goodwin-style situation where you will not be able to get money back off bankers when things go wrong. That is precisely what we have been trying to get away from in Britain."