The White House has declared it will not co-operate with what it termed an "illegitimate" impeachment probe by House of Representative Democrats, setting up a constitutional clash between US President Donald Trump and Congress.

Lawyers for Mr Trump sent a letter to House leaders bluntly stating their refusal to take part in the quickly moving impeachment investigation.

The letter threatens to cease co-operation with Capitol Hill on key oversight matters, accusing lawmakers of formulating their probe "in a manner that violates fundamental fairness and constitutionally mandated due process".

"Given that your inquiry lacks any legitimate constitutional foundation, any pretence of fairness, or even the most elementary due process protections, the Executive Branch cannot be expected to participate in it," White House counsel Pat Cipollone wrote.

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The White House is objecting that the House did not formally vote to begin the impeachment inquiry into Mr Trump, and is also attacking the conduct of House intelligence committee chairman Adam Schiff.

HeraldScotland:

Mr Schiff

House speaker Nancy Pelosi has insisted the House is well within its rules to conduct oversight of the executive branch under the Constitution regardless of a vote.

The letter comes the same day that Mr Trump intensified his fight with Congress by blocking Gordon Sondland, the US European Union ambassador, from testifying behind closed doors about the president's dealings with Ukraine.

Mr Sondland was an intermediary as Mr Trump appealed to Ukrainian President Volodymyr Zelenskiy to investigate political rival Joe Biden and his family.

House Democrats later subpoenaed Mr Sondland, after he failed to show up at a scheduled deposition on Tuesday.

Mr Sondland's lawyer, Robert Luskin, said his client was "profoundly disappointed" that he would not be able to testify.

Mr Schiff said Mr Sondland's no-show was "yet additional strong evidence" of obstruction of Congress by Mr Trump and Secretary of State Mike Pompeo that will only strengthen a possible impeachment case.

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Three House committees are demanding that Mr Sondland appear at a deposition on October 16.

They are also demanding that he produce documents, including communications from his personal device that have been turned over to the State Department but not to Congress.

HeraldScotland:

Mr Schiff

A whistleblower's complaint and text messages released by another envoy portray Mr Sondland as a potentially important witness in the allegations that the Republican president sought to dig up dirt on a Democratic rival in Ukraine and other countries in the name of foreign policy.

The White House has embarked on a newly clear strategy to counter the impeachment threat to Mr Trump: Stall. Obfuscate. Attack. Repeat.

Trump aides have been honing their approach after two weeks of what allies have described as a listless and unfocused response to the impeachment probe.

Ms Pelosi said thwarting the witness testimony on Tuesday was an "abuse of power" in itself by the president.

A senior administration official told reporters that no additional witnesses under its purview will be permitted to appear in front of Congress or comply with document requests, saying the policy under the current circumstances is that the administration will have "a full halt" because "this is not a valid procedure" for an impeachment inquiry.

The White House is claiming that Mr Trump's constitutional rights to cross-examine witnesses and review all evidence in impeachment proceedings extend even to House investigations, not just a potential Senate trial.

It is also calling on Democrats to grant Republicans in the House subpoena power to seek evidence in the president's defence.