A prominent British art dealer who defrauded his clients out of millions by using a scheme that involved masterpieces by artists including Pablo Picassoand Marc Chagall has been sentenced for up to 12 years in prison.

Timothy Sammons – a former Sotheby’s specialist who had offices in New York and London and who brokered multimillion-pound deals for the likes of the Bill and Melinda Gates Foundation – used art that did not belong to him as collateral to obtain personal loans between 2010 and 2015.

Artworks – including Chagall’s Rêverie, Calanque de Canoubier (Pointe de Bamer) by Paul Signac and Picasso’s Buste de Femme – were used to steal between $10m and $30m (£8m and £25m) from clients from the UK, US and New Zealand.

The Manhattan district attorney, Cyrus Vance, said Sammons used his industry experience to gain the trust of prospective art sellers, “then betrayed that trust by pocketing the proceeds of those sales to fund his own lavish lifestyle”.

 “When brokering the sales of high-priced, one-of-a-kind paintings, Timothy Sammons had lying, scamming, and stealing down to a fine art,” Vance said after the sentencing at New York State supreme court on Tuesday.

Sammons was accused of using the proceeds of sales to pay club memberships, credit card bills and first-class travel flights. The judge, Ann Scherzer, said the former dealer “refused to take responsibility or express remorse” and the case was “extremely serious and very disturbing”.

Before sentencing, Sammons said: “I have always said how extremely sorry I am for the trauma I caused.” His solicitor told ArtNet that Sammons “looks forward to getting out of jail, re-establishing his name and paying back his customers”.

Sammons’ company promised to give clients impartial, independent and professional advice on buying, selling and owning art. The dealer, who was previously the head of Sotheby’s Chinese art department, built a reputation – and an estimated £7m fortune – by handling high-profile sales for wealthy clients who traded works by Chagall, René Magritte, Amedeo Modigliani, Picasso and Vincent Van Gogh.

Sammons had his assets frozen in 2015 when the allegations came to light and his £4m London home was repossessed. He will serve his sentence in a New York state prison.

Vance said: “Not only did victims lose millions of dollars, but many lost valuable pieces of artwork that had been in their families for generations.”