Sitting on the toilet, Chris Stringman stared intently at the rows of flashing numbers on the computer screen in front of him.

It was 6.30pm. Downstairs, his partner, Claudia, was cooking dinner after a long day at the primary school where they both worked as teachers.

Now Chris had only 15 minutes before dinner to secretly check the spread bet he had placed on how the Dow Jones industrial average would react to an impending announcement on the US economy.

But as he hid in the loo, Chris didn’t need to look for the news. The figure at the top of his screen showed he’d suffered a loss of £17,000 in one single, brutal hit.

“Instantly, I saw I was dead in the water,” says Chris. “It was my single biggest loss. I couldn’t admit defeat, so desperately I tried to trade my way out of it until Claudia started calling my name and asking what I was doing.”

For three years, Chris hid a gambling habit from Claudia that eventually led to the loss of £130,000.

The opening of his new book, Win. Lose. Repeat, starts starkly: “To my wife and family, I apologise.”

It is not only a morality tale about how spread-betting companies glamorise and draw punters in to a form of gambling that Chris says is no more sophisticated than a game of Play Your Cards Right. It also illustrates how all forms of online gambling have become so effortless that it’s possible to place huge bets on your phone while sitting next to a partner who has no idea you are risking financial ruin for your family.

Chris describes a life of subterfuge: coming to bed late on the pretext of emptying the dishwasher; getting up early so that he could lock himself in the bathroom in order to bet.

Looking back on when he started, Chris, 47, sees that he was an obvious target for spread-betting firms who lure customers with ads on Champions’ League clubs footballers’ shirts and glossy brochures promoting the activity as an exciting way to invest. In fact, he was a dream customer – a single man with his own home and no dependents.

He also had a healthy bank account containing thousands of pounds doing very little. The deciding factor, however, was how he felt about his life.

By 37, he had become disenchanted with his job as a teacher at a school in Hampshire – and was having a form of midlife crisis, desperate to feel more special and successful than he was.

“I was getting to an age when I realised time was running out to be the success I thought I deserved to be,” he says. “This was going to be my short-cut. Gambling gave me hope.”

Spread-betters don’t buy anything or influence markets. They are on the periphery, laying bets on how much prices will rise or fall – a guessing game which Chris says demands no more skill than playing a fruit machine.

“In my mind, this wasn’t gambling. When you spread bet you can have all the graphs, digits and news in front of you. You imagine you’re a City trader, sitting in front of eight screens, shouting ‘Sell!’ You think you’re a master of the universe, when in fact you’re master of nothing.”

The drama and suspense it brought to Chris’s life meant that it quickly became a 24-hour preoccupation.

“I needed to see those numbers constantly. It gave me a false feeling of control. I’d have the screen open during lessons, as I drove. I’d even have a screen propped up on the loo while I was in the shower – and more than once I got out halfway through to make a trade. I didn’t tell anyone. I wasn’t going to until I’d made it big.”

At first, Chris started with small bets – £2 a point. “So if the stock market moves five points and you’re facing in the right direction you’d get a tenner.”

But as the months went on, Chris became increasingly confident: “I have an economics degree and I thought I knew better, which is the fatal flaw for every gambler.”

In the first few months, he won £13,000 on a single trade – almost half his annual salary. Within a year his savings account had suffered a hit of £60,000. But he wasn’t deterred.

Even meeting Claudia, the woman who would become his wife, did not lessen Chris’s need to gamble. “It just changed my justification for doing it. I told myself I was doing it for us.”

When they moved in together, the system was already set up so that Claudia never had reason to be suspicious.

“We kept separate bank accounts, separate computers and phones. All my trading statements were online. It’s incredibly easy to hide.”

Chris found that the toilet became his refuge when he needed to concentrate in private. “It was no different from an alcoholic hiding his gin in the cistern or a coke addict doing a line off the loo seat. [What] makes gambling different is that it’s got no physical symptoms, apart from bad moods.”

Yet five years into his habit, it was becoming clear to Chris that the emotional toll was becoming greater than any financial loss. “I realised it was masochism. I could never relax.”

His fears that gambling would never make him the winner he wanted to be were also brought into even sharper focus by the fact that, around this time, Claudia told him she was pregnant.

“By then, I was having too many apocalyptic days. Any more losses and I knew my child’s future would be in jeopardy. If I stopped then, I knew I could absorb my losses. Any more and I’d have a lot of explaining to do.”

Ultimately, I could just log off. I felt cheated. I had to give up what I believed was my last chance for change

On 29 November 2012, Chris decided he’d had enough. After losing another £5,000 on market betting overnight, the next morning he closed the deal, refunded the remaining deficit to the trading platform, shut the account and went to work.

“Ultimately, I realised I could just log off. I didn’t feel relieved, though, only cheated. I had to give up what I believed was my last chance for change.”

But gradually, over the months that followed, he started feeling lighter, as if a weight had been lifted from his body – and he transferred all the focus he had put into gambling into being a father to his son, Finn, who is now three.

He decided not to tell Claudia what he had done, preferring to process it all by writing a book looking at what buttons gambling pushes.

When he had worked through that aftermath, Chris decided to tell Claudia what he had done. For Claudia, now 33 and a deputy headteacher, it was confirmation of what she had suspected but had never been able to put her finger on – that Chris was holding something back.

She says: “Just before Christmas, when the first copies were being printed, we were sitting in the kitchen and Chris said: ‘You know this book I’ve been writing – it’s about gambling. I used to be a bit of a gambler.’

“I was shocked because I thought it was about his experiences as a teacher moving to Germany [where the couple now live]. Part of me was relieved because sometimes he had been so distracted that I’d half wondered if he’d a wife and child in England. Part of me felt cheated he’d kept it from me for so long. But I also understood that he wanted to wait to write the book because he wanted a positive outcome to come out of this.

“He wanted to be the hero he had tried and failed to be when he was gambling.”

According to the UK markets watchdog, spread betting is form of gambling that has been allowed to hook in vulnerable amateurs for far too long.

Last December, the Financial Conduct Authority proposed to limit how much punters such as Chris could place on certain spread-betting deals, saying that firms allowed punters to take big risks with small stakes, resulting in “rapid, large and unexpected losses”.

For Chris, it’s over, but he’s worried for those still stuck in the habit, who started out wanting to feel alpha-male, but have seen their gambling turn into a guilty secret, emptying their bank accounts and destroying relationships.

“Some will walk away with only a flesh wound. Some will end up with something far more severe,” says Chris. “I have managed to limp away and recover with my relationship and some money left. But unless others find a way to log off, some never will.”

• Win. Lose. Repeat: My Life as a Gambler, from Coin-Pushers to Financial Spread Betting by Chris Stringman (Ortus, £15.58).