David Lammy has accused Cambridge University of a "spin" exercise after some of its leaders published an open letter defending its record on admitting students from diverse backgrounds.
The elite institution posted a statement on its website on Wednesday saying that criticism was "wrong" and "potentially damaging" after Universities Minister Sam Gyimah said the current situation is "staggering".
The letter's authors also said that in its most recent intake, 22% of students enrolled described themselves as black and minority ethnic and 64% were from state schools, which it said were the highest proportions on record.
But Labour MP Mr Lammy, who has investigated intakes at the Oxbridge universities, said Cambridge must "put its money where its mouth is" by encouraging students from wider backgrounds to apply, as well as using contextual data when making offers.
He said: "Instead of looking to blame the applicants, schools and everyone apart from themselves, it would be more helpful if both Cambridge and Oxford focused attention on reforming their admissions processes.
"We need systemic change, not piecemeal PR exercises."
Since his first requests for access data in 2010, Mr Lammy said, "very little has changed" with only "minor progress" witnessed at Cambridge and Oxford.
"Instead of spin and massaging the figures, the universities need to face up to their own institutional failings and ask tough questions about its admissions process," he said.
The letter from Professor Graham Virgo, pro-vice-chancellor for education, Jon Beard, director of Cambridge's admissions office, and Dr Sam Lucy, director of admissions for the Cambridge colleges, came a day after Mr Gyimah's criticisms were published.
After defending recent intakes, they said: "To suggest that no progress is being made in relation to diversity is therefore not only wrong, but potentially damaging and could deter future high-achieving applicants from applying in the first place."
They said that last year Cambridge admitted 58 black students, which admittedly is "very low" as a proportion of its overall intake.
But the "truly shocking statistic", they said, is that it represents a third of all black students who went to university that year having obtained two A-stars and an A at A-level.
"The University of Cambridge cannot single-handedly fix this endemic problem of academic attainment which afflicts all levels of education and society as a whole, reflecting deeper-seated inequalities across the country," they added.
Mr Gyimah told the Daily Telegraph that the institutions had to do more to bring in a students from backgrounds more representative of Britain and said they should "take into account a broad range of factors" rather than just academic results, in what is known as contextual admissions.
Mr Lammy also said contextual data should be used because "the underprivileged kid from a state school in Sunderland or Rochdale who gets straights As is more talented their contemporary with the same grades at Eton or Harrow, and all the academic evidences shows that they fair outshine their peers at university too".