Take That stars Barlow, Howard Donald and Mark Owen are said to have invested a total of £26 million in a tax shelter.
However, Cameron - who has already condemned Jimmy Carr's tax arrangements - chose not to comment on Barlow, who recently received an OBE.
Speaking at a Downing Street press conference today, the PM said: "I am not going to give a running commentary on different people's tax affairs. I don't think that would be right.
"I made an exception because it was a very specific case where the details seemed to have been published and it was a particularly egregious example of an avoidance scheme that seemed to me to be wrong and I made that point."
Shadow Leader of the House of Commons Angela Eagle argued: "Oddly, [Mr Cameron] did not take the opportunity to condemn as morally repugnant the tax avoidance scheme used by Conservative supporter Gary Barlow, who has given a whole new meaning to the phrase Take That.
"If he is also morally repugnant, why has he been given an OBE in the Birthday Honours? Why is the Prime Minister's view of what's dodgy in the tax system so partial? Sir Philip Green has interesting tax arrangements but far from being labelled morally repugnant in a Mexico TV studio, he has got a Government review to head up."
Comedian Jimmy Carr apologised on Twitter for being the biggest beneficiary of the K2 scheme in Jersey.