Mr McEwan is alleged to have “withheld information” from the influential Treasury Select Committee when he denied that he was aware of any criminal activity within the bank by its workers.
But six months later The Times reported that Police Scotland were investigating criminal allegations in relations to a former employee of its RBS Restructuring unit.
Mr McEwan has now defended his comments in a letter to Treasury Committee chairman Nicky Morgan, claiming the activities in question fell outside the period under discussion at the Committee’s January hearing, which related to the now-defunct Global Restructuring Group (GRG).
RBS’ Global Restructuring Group (GRG) has been accused of pushing firms towards failure in the hope of picking up assets on the cheap, but was left off the hook in a recent ruling by the regulator.
The criminal activity being probed by police is said to have taken place between 2014 and 2016.
“We would entirely reject the suggestion that the Committee may have been in any way misled by the evidence that I gave during my appearance before you in January,” Mr McEwan said.
But Ms Morgan said his explanation was “unconvincing”.
“When asked in January if he was aware of any criminal activity at GRG, Mr McEwan withheld information of relevance and interest to the Committee,” she said.
“His letter to me implies that this was not inadvertent, but because he considered that the criminal allegations and police investigation in question were not related to the subject matter of the Committee’s session.
“The Committee is unconvinced by that explanation. It expects clarity and openness from witnesses, and Mr McEwan’s evidence fell short of that standard.”
She also hit out at the tone of the RBS chief executive’s letter saying the Committee is “concerned by the pattern of defensiveness, and a failure to acknowledge mistakes, demonstrated by RBS throughout its handling of the GRG affair.”
“Mr McEwan’s letter to me is an example of this, and it casts doubt on his assurances that RBS’ culture has changed fundamentally since he took up his position five years ago.”
He could now be hauled back to Parliament to face MPs.
“If the Committee decides to ask Mr McEwan to provide further oral evidence, it will expect him to tell the whole truth, not an edited version to suit him,” Ms Morgan said.
Theoretically, anyone seen to be lying in evidence to a select committee could face a substantial fine or imprisonment, though it has rarely been enforced.
Mr McEwan said he was “disappointed” by the Committee’s response.
“It is obviously of the utmost importance to me that the Committee is confident that it can rely on the accuracy of my evidence.
“I replied to the Committee’s questions in good faith and clarified my position in writing.”
He added: “I am confident that when the legal process has run its course, this will be seen to be a unique case.
“There continue to be clear constraints as to what can be disclosed as we would not want to prejudice an ongoing police investigation.”