The man charged with attempting to assassinate Republican presidential candidate and former United States President Donald Trump earlier this month has pleaded not guilty to federal charges.

Ryan Routh, 58, entered the plea to charges that include attempted assassination of a major presidential candidate during a hearing before US Magistrate Judge Bruce Reinhart on Monday.

Wearing a beige prison uniform and shackles on his wrists and ankles, Routh answered “yes, your honour” when the judge asked him if he was aware of the charges against him.

Routh – who also has been charged with assaulting a federal officer and firearms offences – has already been ordered to remain in jail pending trial.

Prosecutors have accused Routh of intending to kill Trump as the former US president golfed at Trump International Golf Club in West Palm Beach, Florida, on September 15.

Routh was arrested after a Secret Service agent saw the barrel of a rifle poking out from brush on the perimetre of the golf course. The agent opened fire and Routh, who fled in a vehicle, was arrested shortly after that.

A struggling roofing contractor, Routh condemned the Republican presidential candidate in a self-published book and dropped off a letter, left months earlier with an associate, referencing an attempted assassination on Trump, according to prosecutors.

“This was an assassination attempt on Donald Trump but I failed you,” the suspect wrote, according to a court filing by prosecutors.

Lawyers for Routh suggested at a September 23 court hearing that the letter may have been an attempt by their client at gaining publicity and highlighted what they called Routh’s efforts to promote democracy in Ukraine and Taiwan.

The Florida incident was the second apparent assassination attempt against Trump in the past few months. In July, the presidential candidate was shot in the ear during a campaign rally in Pennsylvania.

That shooting prompted fierce criticism and questions about the security protocols that were put in place for the event.

Last week, the US Secret Service acknowledged a series of failures it made during the rally, including “deficiencies” in advanced security planning and poor coordination with local police.

“While some members of the advance team were very diligent, there was complacency on the part of others that led to a breach of security protocols,” the agency’s acting director, Ronald Rowe Jr, said on Friday.