A US judge has validated the legality of the Democratic-led impeachment inquiry against President Donald Trump and ordered his administration to hand over an unredacted copy of former special counsel Robert Mueller's report detailing Russian meddling in the 2016 election.

Key points: Judge Beryl Howell gave the Justice Department until next Wednesday (local time) to provide the blacked-out material from the Mueller report

Judge Beryl Howell gave the Justice Department until next Wednesday (local time) to provide the blacked-out material from the Mueller report The US constitution gives the House wide latitude in handling impeachment

The US constitution gives the House wide latitude in handling impeachment A Republican resolution introduced in the Senate criticised the process that House Democrats are using in the impeachment inquiry

US District Judge Beryl Howell, handing a major victory to the Democratic-led House of Representatives, undercut an argument that Mr Trump's fellow Republicans have made in attacking the impeachment inquiry.

The judge said the House need not approve a resolution formally initiating the effort.

The US constitution gives the House wide latitude in handling impeachment. Democrats began the inquiry without putting such a resolution to a vote.

The judge gave the Justice Department (DOJ) until next Wednesday (local time) to provide the blacked-out material from the Mueller report that was subpoenaed by the House Judiciary Committee.

"The reality is that DOJ and the White House have been openly stonewalling the House's efforts to get information by subpoena and by agreement, and the White House has flatly stated that the Administration will not cooperate with congressional requests for information," the judge wrote.

Special counsel Robert Muller's report did not find sufficient evidence of criminal conspiracy. ( AP: Carolyn Kaster )

The department had argued that the redacted information could not be disclosed because it contained material from grand jury proceedings that was required to be kept secret, but the judge strongly disagreed.

"DOJ is wrong," Ms Howell said, adding that the committee's need for disclosure of the materials "is greater than the need for continued secrecy".

"Impeachment based on anything less than all relevant evidence would compromise the public's faith in the process," added Ms Howell, a former federal prosecutor appointed to the bench by Mr Trump's Democratic predecessor Barack Obama.

Ms Howell also ruled that the House has undertaken a legal and legitimate impeachment inquiry and criticised efforts by the Justice Department and the committee's ranking Republican Doug Collins to argue that Democrats had not met the legal threshold.

"Blocking access to evidence collected by a grand jury relevant to an impeachment inquiry, as DOJ urges, undermines the House's ability to carry out its constitutional responsibility with due diligence," the judge added.

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The Democrats sought access to the redacted materials as part of their effort to build a case for removing Mr Trump from office.

The committee, Ms Howell ruled, "has presented sufficient evidence that its investigation has the preliminary purpose of determining whether to recommend articles [of] impeachment," referring to formal charges that the House could approve that would trigger a trial in the Senate on whether to remove Mr Trump from office.

A Republican resolution introduced in the Senate on Thursday criticised the process that House Democrats are using in the impeachment inquiry. It argued that a resolution is needed to initiate such an inquiry. The judge disagreed.

"Even in cases of presidential impeachment, a House resolution has never, in fact, been required to begin an impeachment inquiry," the judge wrote.

'Thoughtful ruling'

Speaker of the House Nancy Pelosi said Congress has the authority to "expose the truth for the American people". ( AP: J Scott Applewhite )

House of Representatives Speaker Nancy Pelosi hailed the judge's order.

"Today's ruling in the US District Court for the District of Columbia is another blow to President Trump's attempt to put himself above the law," Ms Pelosi, a Democrat, said in a statement.

"This critical court ruling affirms Congress's authority to expose the truth for the American people."

Democrat Jerrold Nadler, the panel's chairman, also lauded the ruling.

"The court's thoughtful ruling recognises that our impeachment inquiry fully comports with the constitution and thoroughly rejects the spurious White House claims to the contrary," Mr Nadler said.

Kerri Kupec, a Justice Department spokeswoman, said the department is reviewing the decision.

Mr Mueller submitted his report to US Attorney-General William Barr in March after completing a 22-month investigation that detailed Russia's campaign of hacking and propaganda to boost Mr Trump's candidacy in the 2016 election as well as extensive contacts between the Trump campaign and Moscow.

Attorney-General William Barr was appointed by Mr Trump. ( AP: Patrick Semansky )

But when Mr Barr, a Trump appointee who Democrats have accused of trying to protect Mr Trump politically, made the 448-page report public the following month, some parts were blacked out, or redacted.

Mr Mueller said his investigation found insufficient evidence to establish that Mr Trump and his campaign had engaged in a criminal conspiracy with Russia.

Mr Trump's administration has refused to comply with subpoenas from House committees in the impeachment inquiry seeking documents and testimony. But some current and former administration officials have defied the White House and testified in the impeachment inquiry.

The impeachment inquiry centres not on the Mueller report but on Mr Trump's request that Ukraine investigates a domestic political rival, Democrat Joe Biden.

Reuters