Lord Richard Keen QC, Advocate General for Scotland, faces allegations of behaving in a way that was "likely to diminish the trust and confidence which the public places in a barrister or in the profession".
The professional charge comes after Lord Keen was fined £1,000 in 2017 after pleading guilty to failing to secure a shotgun, which he held a valid licence for.
While investigating the burglary, officers found an unsecured 12-gauge shotgun in a basement in a bag in a cupboard.
Appearing at a Bar Tribunals and Adjudication Service (BTAS) hearing on Tuesday, Lord Keen denied professional misconduct.
Tom Forster QC, representing the Bar Standards Board, said Lord Keen had failed in a core duty of being a barrister by being convicted of the criminal offence which carries a maximum sentence of six months in prison.
Mr Forster said: "I submit, and it brings me no great pleasure to do so, that Lord Keen did breach the core duty.
"First, it is plain that breach of a condition of a shotgun certificate is not a minor criminal offence.
"It may not be a serious criminal conviction but that is not to say it is a minor criminal offence."
Mr Forster said the allegation was not about Lord Keen's integrity or honesty but his "carelessness" with the shotgun which put the public at risk.
"The gun was being kept in a residential property in an urban area therefore there was a heightened obligation to Lord Keen to ensure it was kept securely," he said.
The tribunal heard Lord Keen had used the shotgun on December 27 2016, and forgot to return the gun to a locked safe before going on holiday.
Lord Keen was accused of having a "lacking attitude" to gun safety.
Mr Forster argued that housebreakers including potential burglars could have accessed the weapon.
"I don't say there is any moral turpitude attached to this misconduct," Mr Forster said.
He added: "There is no real substantial reason than 'I forgot'.
"It simply just won't do in these circumstances."
Lord Keen has featured in a number of high profile cases, including representing the Prime Minister at the emergency Supreme Court hearing in September regarding the suspension of Parliament.
Lord Keen, who is also the Ministry of Justice spokesperson for the House of Lords, was made a life peer in June 2015.
He was appointed as Advocate General for Scotland in May 2015, at which time he stepped down as chairman of the Scottish Conservative Party.
However, it was said that Lord Keen is held by the same standards as all members of the Bar Standards Board.
Mr Forster continued: "It's a factor who he is but that does not mean he is treated differently than any other barrister.
"He can't be held to a higher standard than anyone else who is a member of the bar of England and Wales."