Sir Christopher Kelly, chairman of the Committee on Standards in Public Life, spoke after four millionaire Tory donors were controversially handed awards in the New Year's Honours list.
There were knighthoods for hedge fund manager Paul Ruddock and package holiday boss Doug Ellis, and CBEs for investment banker James Lupton and building tycoon James Wates.
Together they have given more than ?1 million to the Tory party.
Sir Christopher wants "big money" donations banned so the honours are not brought into "disrepute". He said: "It implies corruption even when there isn't any."
The watchdog also wants a ?10,000 cap on individual donations.
Shadow Cabinet Office minister Michael Dugher blasted the honours list, saying: "When millions of families are struggling to get by, it's the Tories' friends in the City who get the rewards." Two major Labour donors were also honoured.