Diego Velazquez’s portrait of Cristoforo Segni, assistant to Pope Innocent X, is one of the Spanish artist’s key works.

He created it on a visit to Rome circa 1650, where he also painted his famous portrait of the Pope.

Velazquez painted this portrait in collaboration with painter Pietro Martire Neri

This was one of a number of works Velazquez produced after that hugely important sitting that show various members of the Pope’s retinue.

The canvas also bears the signature of the Cremonese painter Pietro Martire Neri.

It was common for famous painters to task apprentices with producing the simpler parts of their canvases.

However in this case the working relationship seems more collaborative as Neri was already an established painter. This is strange, as Velazquez is not known to have ever worked with another artist.

Debate has raged for centuries over how much input each man had.

Today experts broadly agree that Velazquez painted the head and possibly the collar and sleeves, while Neri was responsible for the rest.

Unfortunately, there’s no record of how the two men came to be introduced so we are unable to get to the bottom of this relationship.

The work is valued at $4m ahead of Sotheby's old masters sale on February 1.

A pair of paintings by the popular Venetian view painter Canaletto are offered as a single lot with an identical valuation.

One shows the churches of the Redentore and San Giacomo, while the other depicts the Bridge of Sighs.

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