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Texan millionaire art collector Richard Drake’s £1.5m claim hangs in the balance this week after a number of the allegations were withdrawn. Last week Mr Justice Buckley reserved his High Court judgment until a later date.

Mr Drake, who bought the portrait of James Stuart, Duke of Lennox, a cousin of King Charles I, from Agnew’s in 1998 believing it to be painted by Sir Anthony Van Dyck, wants his money back after discovering that there were doubts over the provenance.

Mr Drake’s counsel, Joe Smouha, has now withdrawn one of the central planks of their claim – that Agnew’s misrepresented expert views on the painting. However, he insists that the description on the invoice for the painting that it was “by Van Dyck” means that the dealers had a contract to guarantee the authorship of the work, showing that Van Dyck himself had painted the hands, legs and face of the portrait. Unless Agnew’s could convince the judge of this, Mr Smouha argued, Mr Justice Buckley must rule that the painting was not by Van Dyck.

It is this definition of authorship which is now at the centre of the debate, with Agnew’s counsel, Charles Flint QC, telling Mr Justice Buckley: “If you decide that Van Dyck not only composed it but approved it and painted it, then it’s genuine.” He had earlier argued that the burden of proof that the painting was not by Van Dyck lay with the plaintiff.

History shows that many leading artists often painted only the most important parts of the works attributed directly to them, leaving background and less important aspects to their students. So a ruling in favour of Mr Drake could create a precedent preventing those dealing in art from attributing even masterpieces directly to their artists.

A further point of dispute in the case is whether or not the art agent who negotiated the deal on behalf of Mr Drake, Steve Callan, disclosed all the facts he had discovered to Mr Drake at the time.

Mr Flint claimed that Mr Callan’s account of not having access to sales records showing that Agnew’s had bought the painting at auction for £36,000 was “simply not correct”.

Accusing the art agent of dishonesty, he alleged that Mr Callan must have decided not to fully disclose the facts to Mr Drake as it would make the deal “too dangerous”.

But Mr Smouha argued that Mr Drake’s case was not reliant on Mr Callan. Agnew’s had a contract directly with Mr Drake through the sales invoice and it would be up to the art dealers to reclaim any damages awarded against them from Mr Callan, Mr Smouha added.

• Agnew’s face a new claim for compensation, it was reported last week. A British art collector is believed to be suing them for £1m over a landscape painting sold by the art dealers as a Constable in the 1960s which has now been deemed a copy worth about £500.

Despite the length of time since the sale, the claimant is said to be arguing that Agnew’s are liable because they have continued to revalue the painting ever since for insurance purposes.