The White House has notified the House that the Trump administration will not participate in what it is calling Democrats' "illegitimate" impeachment probe.

White House Counsel Pat Cipollone writes in a letter to House Democratic leaders that their inquiry has processed in a "manner that violates fundamental fairness and constitutionally mandated due process" and "lacks any legitimate constitutional foundation" or "pretense of fairness."

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

House Intelligence Committee Chairman Adam Schiff called Sondland's no-show "additional strong evidence" of obstruction of Congress by Trump and Secretary of State Mike Pompeo that will only strengthen Democrats' case.