The guitar George Michael performed with in his 1987 Faith music video has sold for £180,000 at auction, after a row over the sale of memorabilia related to the singer.

The guitar was one of more than 30 items auctioned by Sworders from the collection of Siobhan Bailey, Michael’s long-time personal assistant. The sale fetched a total of £238,000 excluding buyers’ premium.

Bailey died in 2014 and the items were sold by her estranged husband Ian Belcher, who inherited them after her death.

Gerard Bailey, Siobhan’s brother, told the BBC she “would be horrified at the prospect of the auction being held in her name and that her personal, cherished possessions are being sold off to the highest bidder”. Sworders and Belcher declined to comment.

Sworders described the 32 lots, including signed photographs, invitations to events and presentation CDs and discs, as a “remarkable group of memorabilia from the collection of Siobhan Bailey, George Michael’s long-time personal assistant and trusted confidante”.

Michael grew up in Hertfordshire and shot to fame alongside Andrew Ridgeley in pop duo Wham! He went on to enjoy solo success with hits such as Faith, Careless Whisper, Father Figure, Outside and Fastlove. He died in 2016.

Siobhan was in the process of divorcing Belcher when she died, but was legally still married to him. She did not leave a will.

Her brother claimed she had started to draw up a will shortly before she died, which would have left the items to her nieces and nephews, but it was not registered.

There is no suggestion that Belcher did not have the right to sell the items, because her estate legally passed to him when she died.

Bailey said if his sister had lived a few weeks longer, the items would not have passed to Belcher because he would not have been considered her next of kin.

In a letter to the auction house seen by the BBC, Bailey said the rest of the family considered the auction a “breach of [Siobhan’s] privacy”, adding that the use of her name was “disingenuous” and “wholly unethical”.

He told the auction house that Belcher’s “misuse of my sister’s name” was “frankly dishonourable”, and profiteering from the items was “egregiously shameful”.

Bailey asked that the auction should not go ahead, or be in Belcher’s name rather than Siobhan’s.

Siobhan Bailey began working as Michael’s personal assistant in 1984, joining him in the year Wham! had their first number-one hit with Wake Me Up Before You Go-Go.

She remained his PA for 20 years, and her brother told BBC News she avoided the limelight and guarded the pop star’s privacy, despite the attention of tabloid journalists hoping for stories.

“I think she was one of the few people George trusted and whose opinion he valued,” Bailey said.

“She was always honest and straight with him. I guess that’s why she stayed working for him for so long through the very, very good and not so good times.”

The guitar was “a gift” to his sister because the singer “thought so much of her”, he said.

The instrument had a key role to play in Michael’s reinvention from teen pop idol to serious solo artist.

In the 1987 video for Faith, Michael – in ripped blue Levi’s jeans, white T-shirt and black leather biker jacket – strummed the vintage acoustic guitar that he reportedly picked up in a shop in Los Angeles shortly beforehand.

Guitarist magazine recently billed it as “arguably the most famous Gretsch Synchromatic in the world”.

Other items on sale included a note written “with love and admiration” from Michael and Ridgeley, an official Wham! photograph signed “To Siobhan (Special…)”, and an invitation to Ridgeley’s 21st birthday party.

“I know she was proud of the various personalised gold and platinum discs she got, though she was very modest about it all,” Bailey said.

Speaking before Thursday’s auction, he said “she would never have consented” to it, adding that the sale “breaks my heart”.

“I think she’d be devastated that something as personal as her Filofax [tour itinerary] and notes from George are up for grabs. It’s just so tawdry and contrary to how she was,” he said.

Siobhan died suddenly and unexpectedly of natural causes at the age of 56. According to her brother, she wanted the possessions to “have remained with her family and we would never have sold”.

He accused Belcher of deciding “12 years after my sister’s death to cash in”.

Bailey acknowledged that Belcher had a legal right to sell the items but called it “disingenuous” that the sale was being promoted in her name.

Sworders auctioneer Guy Schooling told Guitarist, external that Belcher had been traumatised following Siobhan Bailey’s death and stored all the items in his garage.

“[He was later] persuaded that these things couldn’t stay in the garage forever because they would have deteriorated,” Schooling said.

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