Attorney general William Barr will not testify before a House committee probing the handling of the Mueller report, a senior Democrat has claimed, accusing him of being “terrified”.

In the latest twist to the feud between the Trump administration and Democrats on Capitol Hill seeking to force officials to testify, Democratic congressman Jerry Nadler said Mr Barr would not appear before his committee.

Mr Barr had been due to testify before the House of Representatives’ judiciary committee on Thursday, following his testy appearance before the Senate judiciary committee on Wednesday, where the attorney general clashed with Democrats as he defended his handling of the publication of Robert Mueller’s Russia probe report.

But barely an hour after Mr Barr had completed testifying before senators, Mr Nadler held a press conference to reveal there would be no subsequent appearance on Thursday. The attorney general had declined to appear after Democrats insisted he be questioned not just by senators but by committee lawyers.

“He’s trying to blackmail the committee,” declared Mr Nadler. “The administration cannot dictate the terms of our hearing in our hearing room.”

Mueller investigation: The key figures Show all 12 1 /12 Mueller investigation: The key figures Mueller investigation: The key figures Robert Mueller is the special counsel overseeing the investigation into Russia's meddling in the 2016 election, and potential obstruction of justice by the president. Mr Mueller has a pristine reputation in Washington, where he was previously in charge of the FBI. Throughout his investigation, he and his team have been notoriously tight lipped about what they know and where their investigation has led. REUTERS Mueller investigation: The key figures Former FBI director James Comey was the catalyst that led to the appointment of special counsel Robert Mueller. Mr Comey was fired by the president after Mr Trump reportedly asked him to drop his own Russia investigation. Mr Trump has long maintained that the investigation is a "witch hunt". AFP/Getty Images Mueller investigation: The key figures Deputy attorney general Rod Rosenstein had authority over the special counsel investigation for much of the two years it has been active. Mr Rosenstein found himself with that responsibility after then-attorney general Jeff Sessions recused himself from that oversight. AP Mueller investigation: The key figures Attorney general Jeff Sessions's decision to recuse himself from oversight of the special counsel investigation may have cost him his job in the end. Mr Sessions resigned last year, after weathering a contentious relationship with Donald Trump who vocally criticised his attorney general for taking a step back. Mr Sessions recused himself from the oversight citing longstanding Justice Department rules to not be involved in investigations overseeing campaigns that officials were apart of. AP Mueller investigation: The key figures Attorney General William Barr is currently responsible for oversight of the special counsel investigation. Mr Barr's office will be the first to receive the Mueller report when it is finished. His office will then determine what portion or version of that report should be delivered to Congress, and also made public. EPA Mueller investigation: The key figures Michal Cohn is the president's former personal lawyer, who has been helping the special counsel investigation as a part of a plea deal over financial crimes, and campaign finance crimes, he has pleaded guilty to. Among those crimes, Cohen admitted to facilitating $130,000 in hush money payments to adult film actress Stormy Daniels during the 2016 campaign. Cohen has said he did so at the direction of Mr Trump. Cohen has also admitted that he maintained contacts with Russian officials about a potential Trump real estate project in Moscow for months longer than Mr Trump and others admitted. The talks continued well into 2016 during the campaign, he has said. AP Mueller investigation: The key figures Stormy Daniels has alleged that she had an affair with Donald Trump in 2006, soon after Melania Trump gave birth to Baron Trump. The accusation is of particular importance as a result of the $130,000 hush money payment she received to keep quiet about the affair during the 2016 campaign. AP Mueller investigation: The key figures Paul Manafort was Donald Trump's former campaign chairman. Manafort was charged alongside Rick Gates for a slew of financial crimes, and was convicted on several counts in a Virginia court. He then pleaded guilty to separate charges filed in a Washington court. Manafort has been sentenced to just 7.5 years in prison for his crimes — in spite of recommendations from the special counsel's office for a much harsher sentence. AP Mueller investigation: The key figures George Papadopoulos was one of the first individuals associated with the Trump campaign to be charged by the Mueller probe. He ultimately received a 14 day prison sentence for lying to investigators about contacts he had with Russian officials. AP Mueller investigation: The key figures Roger Stone is a well known political fixer and operative, who has made a name for himself for some dirty tactics. He has been charged by the Mueller probe earlier this year, and he has been said to have had prior knowledge that WikiLeaks planned on publishing stolen emails from the Hillary Clinton campaign in 2016. Getty Images Mueller investigation: The key figures Rick Gates was charged alongside former Trump campaign chairman Paul Manafort for a range of crimes. Gates, who worked alongside Manafort for a pro-Russia Ukrainian political party. The two were charged with conspiracy and financial crimes. Gates pleaded guilty. AP Mueller investigation: The key figures Former national security adviser Michael Flynn was one of the first casualties of the Russia scandal, and was forced out of his position in the White House weeks after Donald Trump took office. Flynn pleaded guilty in 2017 to "willfully" making fraudulent statements about contacts he had with Russian officials including former Russian ambassador to the US Sergey Kislyak. Flynn then lied to Vice President Mike Pence about that contact. REUTERS

Mr Nadler, who also said the department of justice had informed his committee it would not comply with its subpoena for the full, unredacted report from the special counsel, did not rule out issuing another subpoena to try and force Mr Barr to appear on Thursday. He said he hoped Mr Barr would reconsider his position overnight.

The department of justice confirmed Mr Barr would not appear and claimed Mr Nadler had put “unprecedented and unnecessary” conditions about his testimony.

“Congress and the executive branch are co-equal branches of government, and each have a constitutional obligation to respect and accommodate one another’s legitimate interests,” said department spokesperson Kerri Kupec.

The decision by Mr Barr is just the latest tussle between the White House and Democrats, who now control the House. While Democrats, disappointed that Mr Mueller’s report did not include a smoking gun to bring down the president, are determined to question under oath his top officials, Mr Trump is determined to try and ensure they do not.

“There is no reason to go any further, and especially in Congress, where it’s very partisan – obviously very partisan,” Mr Trump recently told the Washington Post. “I don’t want people testifying to a party, because that is what they’re doing if they do this.”

Nancy Pelosi claims William Barr has 'gone off the rails' after attorney general claims US intelligence spied on Trump campaign

The appearance of Mr Barr came shortly after the emergence of a letter from Mr Mueller, sent to the attorney general, in which he outlined complaints about the four-page summary that had been presented to Congress about his report.

“The summary letter the department sent to Congress and released to the public late in the afternoon of March 24 did not fully capture the context, nature, and substance of this office’s work and conclusions,” Mr Mueller wrote.

In his summary, Mr Barr said that while Mr Mueller had found no evidence the Trump campaign colluded with Russia, he had not reached a decision about the claims of obstruction of justice.

When a redacted version of the report was subsequently released, it emerge that Mr Mueller had uncovered numerous connections between members of the Trump campaign and Russia. Mr Mueller also wrote “while this report does not conclude that the president committed a crime, it also does not exonerate him”.

Mr Barr, during his testimony, stood by his determination not to charge Mr Trump for obstruction — arguing that, since there was no collusion or conspiracy, that the president could not have obstructed justice by firing former FBI director James Comey and then repeatedly attempt to get others to fire Mr Mueller.

Pushed on whether it was appropriate for Mr Trump to lie to the American people about contacts between his campaign and Russians, about his intentions with regards to Mr Mueller’s employment as special counsel, and other questionable instances surfaced by the report, Mr Barr said that his job is not to determine who was behaving well or not.

“I’m not in the business of determining wether lies were told to the American people,” Mr Barr said.“I’m in the business of determining whether crimes were committed.”