Official figures show that the outstanding student-loan book was £121 billion by March

Labour has opened the door to writing off billions of pounds of student loans as part of an overhaul of higher-education funding.

The party has pledged to abolish student fees and bring back maintenance grants, and John McDonnell suggested today that the party could go further by writing off existing debt.

It came as the shadow chancellor used a speech to accuse Boris Johnson of allowing the country’s “super-rich” to buy access to “tax breaks and corporate giveaways” by giving money to the Conservatives.

With Labour launching its manifesto on Thursday, Mr McDonnell hinted that the party could address the student-loan book, saying that the system was unsustainable.

“The system was designed in such a way that it would not work and it would eventually