The US Supreme Court has heard arguments in back-to-back cases that could determine the fate of more than 40 million Americans’ student loans.

It comes after President Joe Biden, a Democrat, announced a plan last year to forgive up to $10,000 (£8,400) in federal student loans per borrower.

So far, Republican-appointed lower court judges have blocked the plan.

Top court justices on Tuesday questioned if the president’s actions were lawful and warranted.

The Supreme Court, which has a 6-3 conservative majority, will make a final ruling on cases by the end of June.

The Biden administration argues that, under a 2003 law known as the Higher Education Relief Opportunities for Students Act or Heroes Act, it has the power to “waive or modify” loan provisions to protect borrowers affected by “a war or other military operation or national emergencies”.

Government lawyers were pushed during Tuesday’s arguments on that broad interpretation of the law.

“We’re talking about half a trillion dollars and 43 million Americans. How does that fit under the normal understanding of modified?” Chief Justice John Roberts asked US Solicitor General Elizabeth Prelogar.

Mrs Prelogar revised that, in the given context, “modify” could involve making broad changes to protect borrowers. She added that the administration was not seeking to assert regulatory authority but to implement a benefits programme.

Without the plan in place, “defaults and delinquencies will surge”, he said.

The twin cases on the court docket hinge on whether challengers to the loan forgiveness plan can demonstrate they would be harmed by the programme. If so, they would have the “standing”, or the legal right, to sue and prevent the plan from being implemented in its current form.

The first case is a challenge from six Republican-led states, including Missouri.

Plaintiffs in the first suit argue that Missouri’s federal loan servicer – the Missouri Higher Education Loan Authority (Mohela) – will suffer financially as it would no longer receive millions of dollars in fees for loans forgiven by the program.

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But at least two justices appeared skeptical, including liberal Justice Elena Kagan, who pushed the attorney arguing in the suit, Nebraska Solicitor General James Campbell, why he was arguing on behalf of Missouri.

“Usually, we don’t allow one person to step into another’s shoes and say, ‘I think that person suffered harm,'” she said. “So why isn’t Mohela responsible for deciding whether to bring this suit?”

The second lawsuit involved two student loan borrowers, Myra Brown and Alexander Taylor. Ms Brown did not qualify for forgiveness, while Mr Taylor did not qualify for the $20,000 maximum.

Conservative justices focused on whether it was appropriate for some Americans to receive debt relief but not others, with Justice Samuel Alito asking: “Why is it fair? Why was it fair to the people who didn’t get arguably comparable relief?”

But the court’s liberal justices pushed back. Justice Sonia Sotomayor noted that “different people got different benefits because they are qualified under different programs”, while Biden appointed Ketanji Brown Jackson claimed “the same fairness issue would arise with respect to any federal benefit programs”.

Student loan repayments were initially paused under the Trump administration in 2020 because of the coronavirus pandemic.

Mr. Biden kept that pause in place until he decided, in August, to eliminate more than $400bn in debt – up to $10,000 each for those earning less than $125,000, and up to $20,000 for students on need-based Pell Grants.

The White House estimates that nearly 90% of the country’s student borrowers will qualify for relief under its plan, and about 26 million people have already applied for forgiveness. But relief is on hold as legal challenges to the plan make their way through the courts.

Ahead of the hearing, hundreds of activists braved the cold and rain to participate in rallies outside the Supreme Court on Monday and Tuesday.

They were joined by some of the Democratic Party’s most outspoken figures on student debt. Congresswoman Ayanna Pressley said opponents of loan forgiveness were “disconnected from the hardships of everyday folks” while Senator Elizabeth Warren warned “an extremist court” may take away the opportunity “to build more secure futures”.

The White House has not yet said what it will do if justices strike down its plan.

Mr. Biden said Monday that he is “confident the legal authority to carry out that plan is there. I promise you: I have your back.”