A federal judge has ruled that former US vice president Mike Pence will have to testify before a grand jury as part of the investigation into efforts by former president Donald Trump to overturn the results of the 2020 election.

That is according to two people familiar with the decision, who spoke on Tuesday on condition of anonymity because it remains under seal.

The sources said, however, that Mr Pence would not have to answer questions about his actions on January 6 2021, when a mob of Trump’s supporters stormed the Capitol building as Pence was presiding over a joint session of Congress to certify Democrat Joe Biden’s victory.

But he would have to testify about any potential illegal acts committed by the former president, one of the insiders said.

Mr Pence and his attorneys had cited constitutional grounds in challenging the grand jury’s subpoena.

They argued that, because he was serving in his capacity as president of the Senate that day, he was protected from being forced to testify under the Constitution’s “speech or debate” clause, which is intended to protect members of Congress from questioning about official legislative acts.

Mr Trump’s lawyers had also objected to Mr Pence complying, citing executive privilege.

A spokesman for Mr Trump criticised the decision in a statement, accusing the Justice Department of “continuously stepping far outside the standard norms in attempting to destroy the long accepted, long held, constitutionally based standards of attorney-client privilege and executive privilege”.

“There is no factual or legal basis or substance to any case against president Trump,” they added, but did not respond to questions about how Trump’s legal team might respond.

Mr Pence’s team is evaluating whether it will appeal.

The sealed ruling from US District Judge James Boasberg sets up the unprecedented scenario of a former vice president being compelled to give potentially damaging testimony against the president he once served.

And it comes as Mr Pence has been inching closer to announcing a run for the 2024 Republican presidential nomination — a decision that would put him in direct competition with his former boss.

Mr Pence was subpoenaed earlier this year to appear before the grand jury in Washington investigating election interference.

In public appearances, he has cast the that action as unconstitutional and unprecedented and said he would pursue the matter as far as the Supreme Court.

Still, he told ABC’s This Week that he might be open to testifying about matters not directly related to his congressional role.

“I’ve actually never asserted that other matters unrelated to January 6 would otherwise be protected by speech and debate,” he told the network. “We’re gonna make that case, but I promise you we’ll respect the decisions of the court.”

A Justice Department special counsel, Jack Smith, is investigating attempts by Mr Trump and his allies to overturn Mr Biden’s win in the 2020 presidential election to keep Mr Trump in the White House.

Multiple aides of Mr Trump have already appeared before the federal grand jury, as well as a separate Georgia panel examining allegations of Mr Trump mishandling classified documents at his Mar-a-Lago club in Florida.

A spokesman for the special counsel’s office declined to comment.

Mr Pence has spoken extensively about Mr Trump’s pressure campaign urging him to reject Mr Biden’s victory in the days leading up to January 6, including in his book, So Help Me God.

Mr Pence, as vice president, had a ceremonial role overseeing the counting of the Electoral College vote, but did not have the power to impact the results despite Mr Trump’s contention otherwise.

Mr Pence has said that Mr Trump endangered his family and everyone else who was at the Capitol that day and history will hold him “accountable”.