Phil Willis has warned that "inconsistency in standards is rife"
Universities in England should "get real" over safeguarding the quality of degrees, says Phil Willis, the chair of the universities select committee.
Mr Willis said they should explain why the number of first class degrees had almost doubled since the mid-1990s.
Wendy Piatt of the Russell Group of leading universities said it reflected higher achievements at A-level.
A report published by Mr Willis's committee called for a radical overhaul of the university standards watchdog.
The cross-party committee of MPs had warned in a report published on Sunday that "inconsistency in standards is rife" and that universities should be able to show the relative value of degree grades.
Speaking on BBC Radio 4's Today programme, Mr Willis cast doubt on the rapid increase in the proportion of top grades.
The report showed that in 1996-97, 7.7% of degrees had been first class. In 2007-08 it was 13.3%.
The proportion of lower seconds and third class degrees had fallen across this period - and the report noted that between 1994 and 2002 the number of higher grades "tended to rise in almost all subject areas".
"Every year when A-level results come out there is a cry from universities that standards are getting poorer ... so how can it be that in 15 years we've got twice as many people achieving the very highest levels?" asked Mr Willis.
Dr Piatt said that for Russell Group universities there had not been any compromise on standards and that the Quality Assurance Agency provided an adequate protection.
"The increase in success rates for our graduates should be applauded and it does reflect very closely the fact they have achieved more at A-level and the equivalent," she said.
"And I don't think we should be too worried about that."
The MPs' report had accused the university sector of being "defensive and complacent" about the value of degrees - and Mr Willis once again challenged the credibility of the current watchdog.
"This is the sort of complacency that we talk about in our report. The Quality Assurance Agency is funded by the universities simply to inspect their own quality assurance systems.
"It doesn't make the sort of comparisons between universities, between courses, it doesn't look at the quality of teaching, it doesn't look at anything involved in the process of giving undergraduates the education they need to go out into a competitive economy.
"Let's get real, let's look at our system," said Mr Willis.
"There is no complacency," said Dr Piatt.
But she said the problem facing the sector was not about assessing quality but ensuring that applicants, including the least well off, had enough information about what courses were available.
"The problem is in getting the proper information, advice and guidance to students, so they know which course they should be taking at which institution," she said.