Donald Trump intends to assert executive privilege in a congressional investigation into the January 6 insurrection at the Capitol, a move that could prevent the testimony of former aides, according to a letter on behalf of the ex-president.

The letter went to at least some witnesses who were subpoenaed by the House committee and it makes clear Mr Trump plans to invoke privileges meant to protect presidential communications from being shared with Congress. The substance of the letter from a lawyer for Mr Trump was described to The Associated Press on Thursday by a person familiar with the letter who spoke on condition of anonymity.

Mr Trump said in a statement last month that he would “fight the Subpoenas on Executive Privilege and other grounds, for the good of our Country”.

The move sets the stage for a likely clash with House Democrats investigating the roles of Mr Trump and his allies in the run-up to the riot, when a large mob of Trump supporters broke into the Capitol as Congress was certifying the results of the presidential election won by Democrat Joe Biden.

The committee is rapidly issuing subpoenas to individuals who are either connected to Mr Trump or who helped plan the massive rally on the morning of January 6 at which he told his supporters to “fight like hell”.

The committee, which was formed over the summer, last month issued subpoenas to Mark Meadows, Mr Trump’s former chief of staff, Dan Scavino, the former deputy chief of staff for communications, Kashyap Patel, a former Defence Department official, and Steve Bannon, a former Trump adviser.

It was not immediately clear how those witnesses would respond to Mr Trump’s action or what consequences they might face if they refuse to cooperate.

Mr Patel said in a statement he would “continue to tell the American people the truth about January 6″. The statement did not say whether he would comply.