Pirc, which represents the interests of fund managers and pension funds with more than 1.5 trillion pounds ($2.4 trillion) of assets, said the complexity of the Barclays' plan "prevents analysis of the design and quantum of executive pay."
Pirc also slammed the British bank's choice of performance targets that link pay to its capital ratio.
"This is not appropriate, particularly in a regulated bank where regulatory requirements are being used to justify incentive payments," Pirc said in its weekly newsletter.
Barclays has linked pay for senior staff to its core Tier 1 capital adequacy ratio, with bonuses deferred over three years and clawed back if the ratio falls below 7 percent.
Criticism over pay in the banking industry has refused to go away two years on from the financial crisis, despite changes to the way many executives are paid, as bumper pay packages have returned for some.
Barclays investors will vote on its remuneration report at its annual general meeting on April 27.
(Reporting by Tommy Wilkes; Editing by Will Waterman)