More than 100 MPs have written to Oxford and Cambridge urging the universities to take urgent action to recruit more talented students from disadvantaged and under-represented backgrounds.
In an open letter addressed to the vice chancellors of the prestigious institutions, the group say they are "gravely concerned" that the proportion of offers made to those from the richest homes has increased in recent years, and warns that "elite universities must not be elitist".
The letter comes after Freedom of Information requests by Labour MP David Lammy found that Oxbridge's populations are still heavily made up of white, well-off students from the South of England.
In a response to the letter, organised by Mr Lammy and signed by MPs including shadow education secretary Angela Rayner and former Labour cabinet minister Yvette Cooper, Cambridge Vice Chancellor Professor Stephen Toope said the university recognises that "more hard work is required" but that much has already changed over the last decade.
Oxford said its vice chancellor, Professor Louise Richardson, would respond in detail in the coming days and that it shares concerns about "uneven access" to the institution, the Guardian newspaper reported.
The letter, signed by 108 MPs, says: "We are united in our acknowledgement of and support for the outstanding education that your universities provide, but our elite universities must not be elitist, drawing up the ladder to success and reinforcing privilege."
It adds: "It is of grave concern that the proportion of offers made to applicants from the top two social classes has increased since 2010.
"The Universities of Oxford and Cambridge receive over £800 million each year directly from the taxes paid by our constituents, including the huge swathes of the country that are hugely under-represented with regards to the number of offers made to students from Oxbridge."
The letter warns: "Much more work is required to find the most talented students who may be from disadvantaged backgrounds, lack the confidence or support networks to apply to Oxbridge or live in parts of the country and attend schools that do not traditionally send many students to Oxbridge.
"We call on you to take the initiative in directly contacting talented and straight A students, whether following GCSE, AS-level or A-level results, and in reaching out to parts of our society and our country that are under-represented."
"An Oxbridge education is still seen as a golden ticket into a top job and the Universities of Oxford and Cambridge - supported by significant taxpayer funding - still hold the key to a job in our top professions," the letter concludes.
"Whilst we do not doubt that you are aware of your responsibility to ensure that the most talented students, regardless of their background, apply and are offered a place to study at our top universities, we call on you to set out exactly what steps you will be taking to address this situation and make good on this responsibility."
In his response, Prof Toope said: "I believe that all universities, including Cambridge, have a duty to be open to people from all backgrounds, irrespective of race, class and origin."
An Oxford spokesman told the Guardian: "We share the concerns expressed by MPs about uneven access to Oxford, and know we have a great deal of work to do. We are committing more money and people than ever before to addressing these problems."