Sunderland’s players had to be talked out of a remarkable stunt aimed at goading Newcastle United’s supporters during one of last season’s Tyne-Wear derbies, a new book claims.

The Sunderland team – then managed by the England manager, Sam Allardyce – are said, according to claims in the book, to have asked for T-shirts with a specially designed take on the clubs’ nicknames, showing a black cat tearing apart a magpie.

The idea, it is claimed, was to wear the T-shirts beneath their kit and if they won – which Sunderland did, 3-0, in the fixture at the Stadium of Light – remove their tops before running towards the Newcastle fans to poke fun at their rivals.

The claims – disputed by Sunderland – are made in a book written by the football agent Jon Smith, The Deal: Inside the World of a Super-Agent, in a passage where he discusses some of the unseen work agents do to help footballers stay out of trouble.

Sunderland’s players were so determined to go through with the plan, he claims, they were even willing to risk possible fines or the club facing disciplinary action when attempts were made to talk them out of it. However, the team were dissuaded from going through with it because of the potential repercussions, according to Smith.

“Before one of the derbies in the 2015-16 season, the Sunderland players asked an agent to produce a T-shirt depicting a black cat savaging a magpie,” he writes. “They were going to wear it under their kit during the game and at full-time, presuming they won, all take their tops off and run towards Newcastle supporters showing off these T-shirts.

“In the overall scheme of things, who cares? They aren’t necessarily breaking the law but in football emotion runs so high it could create a disturbance or even a riot. And in that riot, if one child gets hurt, it is appalling. If one child dies, it is a tragedy. That’s what we have to contend with.

“It took that agent six days to talk the Sunderland squad out of it. He told them they would be fined hundreds of thousands of pounds but they didn’t care; there is at least something to be said for players being so impassioned by the cause that a financial punishment did not deter them, especially given the prevailing perception of footballers being driven solely by money. Here was the antithesis – they wanted to sacrifice money in a public show of hunger to avoid relegation.”

Sunderland’s version of events is that there was only one player involved and it was said in jest, but Smith told the Guardian: “This is one of those circumstances where it was between the players and the agent, rather than the club.”