It works like this: when a customer opens an account, they might be given a stake factor of 1, meaning they can bet 100% of the normal maximum stake, for example £500.

“As soon as people start winning or losing, that gets adjusted,” said Cameron, formerly of William Hill. “It starts with 50% and if they keep doing it [beating the bookie], it’ll keep going down. At William Hill it went down to 25% to 10% and eventually down to 1%.”

One gambler, Bernard Henry, shared the results of a demand sent to Coral to see what data it held on him, known as a subject access request.

“He’s allowed to lose 40 times what anyone else was and it was astronomical what he was punting. There was no conscience to it.”

It showed he had won just £38 over four years but had been prevented from placing sports bets after the bookie realised he had beaten their odds 73% of the time. He was still allowed to bet on casino products where the house never loses over time.

Henry believes bookmakers shared data about him with other operators, via software that companies are permitted to use which is intended to prevent fraud but might also be used to shut down a smart better.

Everyone the Guardian spoke to said accounts were rarely closed completely, for good reason.

According to Paddy Power’s manual, “warm” customers are “of use to us as marks”, meaning they are worth monitoring to assess if your prices are wrong. Even arbers, seen as cheats in the industry, would be set at a factor of 0.01.

One former employee of Betfair believes this is about satisfying City investors, who see customer numbers as one measurement of success. “If they close all the bad customers, you go from 50,000 active users to 10,000,” he said.

The flipside of factoring down winners is that losers have, over the years, been given more leeway to place ever bigger bets.

The biggest stake factor Rory recalled a customer being allotted was 40. “He’s allowed to lose 40 times what anyone else was and it was astronomical what he was punting. There was no conscience to it.”

One former employee of Betfair believes this is about satisfying City investors, who see customer numbers as one measurement of success. “If they close all the bad customers, you go from 50,000 active users to 10,000,” he said.

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