The news comes as angry shareholders overwhelmingly voted against the bank's remuneration report in protest at the former chief executive's package.
The vote, at the RBS group's annual general meeting in Edinburgh, is not legally binding and has no impact on Sir Fred's pension.
Sir Fred is understood to have told the bank he would not hand back any of the pension, despite assurances earlier today from the bank's chairman that he was thinking about whether to make a reduction.
New RBS chairman Sir Philip Hampton maintained ahead of the bank's AGM that he had asked Sir Fred about such a move only weeks ago and that the former boss was considering it.
The embattled banking group, which is now majority-owned by the Government, announced earlier that the 2,700 staff previously earmarked to be axed were "not the end of the story".
Sir Philip faced angry questions from shareholders at the Edinburgh International Conference Centre today.
Shareholders called for more change - from firing more executives to ridding RBS of its workplace "culture of fear".
Sir Fred's legacy drew much of the criticism, with one shareholder branding him a benefits scrounger.
Describing RBS as a "dead bank on life-support", the shareholder added: "He's a benefit scrounger on a massive scale.
"When a benefit scrounger is caught out he has to pay it back - it has to be payback time."
Sir Philip told the audience that the former boss's pension contract was legal but added: "We're looking at that contract to see if there are any opportunities for redress."
Shareholder John Waterson voiced "grave reservations" about the make-up of the present board - particularly Gordon Pell and Guy Whittaker, who were both on the bank's previous board.
"It begs the question: why are these two individual gentlemen still on the board of directors at Royal Bank of Scotland?" Mr Waterson asked, to applause from shareholders in the audience.
He added: "Far from continuing with them as members of the board - I am sure I speak for a lot of people in this audience - all of those board members should be in jail."
Sir Philip told the packed meeting that 12 board members had departed since October last year and Mr Whittaker and Mr Pell have his "total confidence".
He added: "You can't think you can get rid of everyone and run the business - that's just not prudent."Reuse content