Hunter Biden, the son of President Joe Biden, has entered a not guilty plea to charges of illegally owning a handgun at a Delaware courthouse.

Prosecutors allege that Mr Biden, 53, lied about his drug use on application forms when he purchased the weapon in 2018.

Mr Biden has acknowledged that he was a heavy user of crack cocaine at the time, but denies breaking the law.

He faces up to 25 years in prison if convicted of the three federal counts.

The charges could also lead to fines of up to $750,000 (£621,000).

Clad in a dark suit and flanked by Secret Service agents and his defence team, Mr Biden did not speak during the brief arraignment, other than to acknowledge the charges against him and his rights as a defendant.

The plea was entered on his behalf by his attorney, Abbe Lowell. In a statement afterwards, Mr Lowell said he believes the charges are the result of "political pressure" by right-wing Republicans.

Mr Biden appeared to be relaxed, laughing and smiling with his defence team before proceedings began and waving to an acquaintance in the room. As he exited, he waved to reporters and said "thank you".

While no date has been set for the beginning of the trial, prosecutors and Mr Biden's defence team have been given a 30-day deadline to file any pretrial motions they might have.

As part of the judge's conditions for his remaining free pending trial, he was ordered to abstain from drugs and alcohol, seek employment and clear any travel with a probation officer.

A proposed plea deal to resolve the charges abruptly fell apart in July, meaning Mr Biden was indicted shortly after.

He was charged with two counts of making false statements and one count of illegal gun possession. The charges all relate to his purchase of a revolver at a Delaware gun store in October 2018, which he kept for around 11 days.

By Mr Biden's own admission - published in a 2021 memoir - he was in the throes of a "full-blown addiction" at the time.

Two of the criminal counts against him, each punishable by up to 10 years, stem from the allegations that Mr Biden lied about his drug use on the forms. A third count, related to his possession of a firearm while a drug user, is punishable by up to five years.

Kevin McMunigal, a former federal prosecutor who now teaches law at Case Western Reserve University in Ohio, said that while the basic facts of the prosecution's case "will be quite easy to establish", they by no means ensure a successful prosecution.

For example, Mr McMunigal noted that all three charges require that Mr Biden was "using or addicted" to drugs when he filled out the form on 12 October 2018.

"Apparently he was in and out of drug rehabilitation during that year....a question may arise as to whether he was using or addicted at that point in time," he said. "This could be tricky for the prosecution to prove if he claims he was clean during the key time period in mid-October".

Additionally, Mr McMunigal said that Mr Biden's lack of a criminal record "is a major factor in his favour in terms of avoiding jail time", and that the non-violent and victimless nature of the allegations "make prison time less likely".

In court, Mr Lowell said that Mr Biden's defence team would seek to have the charges dismissed, arguing that they are both unconstitutional and barred by the previous agreement made with prosecutors.

The argument over the constitutionality of one of the charges - a ban on gun possession for drug users - rests on a Supreme Court ruling that expanded gun rights last year.

In the ruling, the conservative-leaning court said that firearms restrictions must be consistent with the "historical tradition of firearm regulation" in the US.

Lauryn Gouldin, a law professor at Syracuse University, told the BBC that under that criteria, the court would "likely" find the charge against Mr Biden unconstitutional.

In early November, the Supreme Court will also hear another case in which it will hear arguments over whether the government can prohibit gun ownership by people with domestic violence restraining orders.

The outcome of that case, she added, "is likely to be a very clear signal" for the Biden case.

Mr Biden appeared on Tuesday in a federal courthouse in Wilmington, Delaware - which is the hometown of the Bidens.

Security was tight during the brief court appearance in Wilmington, which lasted less than 30 minutes. Mr Biden was escorted into the courtroom by Secret Service agents, while outside, police used dogs to inspect bags and nearby bushes.

One protester - dressed in a striped prison costume - stood outside the courthouse holding a sign that read "lock Biden up", occasionally eliciting profanities from passing motorists.

The gun at the centre of the case was found by Hallie Biden, the widow of Hunter's brother Beau, in his vehicle. Ms Biden threw the weapon into a rubbish bin, reportedly because she feared he might use it to hurt himself.

It was later discovered and returned to the store, but not before it prompted separate investigations by both Delaware police and the US Secret Service.

In June, a two-part agreement was reached between prosecutors and Hunter Biden's legal team in which he agreed to admit illegal possession of a firearm and undertake addiction treatment and monitoring. Under that agreement, he would also be charged with two misdemeanour counts for failing to pay his taxes on time in 2017 and 2018.

But the deal, which would have allowed Mr Biden to avoid felony charges and potential imprisonment, fell apart in July. The judge in the case, Maryellen Noreika, said she could not "rubber stamp" the agreement. She also called the deal's proposed resolution of the gun charge "unusual".

While the tax charges were dismissed in August, prosecutors are expected to refile the charges, or file new ones, in Washington DC or California.

Earlier in September, Mr Biden filed a lawsuit against the Internal Revenue Service, alleging that two of its agents "sought to target and embarrass" him by sharing private tax information.

The case could mean that Hunter Biden faces a criminal trial while his father campaigns for re-election as president.