In 2002, I received a payment of £50,000 from a football agency. It was paid as an advance of earnings they believed they would make if I gave them sole rights to represent me in my career. They said it was a sign of their commitment to my long-term development.

I hadn't heard of this happening anywhere before so I sought assurances from my club manager that nothing untoward was taking place. Apparently, it was not uncommon at the time so I signed the standard two-year agreement and thought nothing more of it.

The following year, the morning after my retirement because of injury was announced, I got a call from from the firm saying I would be sued for loss of earnings if I did not repay that sum in full. By retiring, I had denied them the right to earn back the money they believed was rightfully theirs. Sure, they were sorry for what I was going through, but this was business.

After I consulted the Professional Footballers' Association (PFA) for advice and stalled on a response, they threatened me further if I didn't pay up. I eventually paid back £22,000 and regretted ever going near them in the first place.

Two years ago, former agent Peter Harrison turned whistle-blower and spoke openly about the realities of life in the football industry and the behaviour of those within it. Having previously been the feature of a 2006 Panorama documentary titled 'Undercover: Football's Dirty Secrets', he spoke about special arrangements between clubs and agents and the widespread practice of bungs and illegal approaches. "It's like a magic circle of trust and people don't want to be on the outside," he said at the time.

Last week I met with two agents to discover what changes, if any, had taken place since then. The only way to ensure their honesty was to guarantee their anonymity.

The two agents began our conversation by outlining the process of paying backhanders to agents acting on behalf of club officials. 'middle men' was the term they used for those who need to be paid by a player's agent in order for a transfer to be completed. It is not always clear who they are middle men for, however. "And we're talking here about it happening at every level," said one of the agents.

Once contacted by a middle man, an agent has a decision to make: either the agent can refuse to pay, which may jeopardise the transfer altogether, or lose a considerable amount of the commission by conceding to the demand. "It puts fantastic pressure on your relationship with the player. If a player is offered the opportunity to go to a club that he would walk to to get to, but the only way he's going to get there is through another man, a middle man, it makes it very difficult for you if you've got anything about you as an agent to deny a player."

As can often happen, players can look to squeeze their agent out of the negotiations knowing the middle man is the one holding the power. "And then what do you do? Sue the player?" Naming and shaming wouldn't work either because, as one put it, "for someone to break ranks would be the end of their career in this industry".

The agents I met with gave two examples they had been involved with, both involving Premier League clubs. Once all the negotiations for both transfers had been completed, a call came from an agent acting on behalf of the buying manager. The instruction was simple: they had to give a slice of their earnings to the manager or the deal would fall through.

On the first occasion the issue was resolved with prompt payment. "You buckle to the inevitable on that one. We had to pay [the manager's] man money. We lost out, but we couldn't deny [the player] the move."

In the second case, the player involved refused to pay and stood his ground on a point of principle. The deal still went through but the communication changed from a verbal request to a physical threat by people linked to the agent of the manager who had sanctioned the deal. "They said to [the player], 'you gotta pay it or otherwise it's going to get awkward'." He softened his stance when he was told his knees would be broken. "It went down as scouting expenses. It was £50,000 of the pot that we should have got."

I wondered how common this is. "I think in at least half of our [firm's] transfers that would be the case, yes." The agents named one club they know of where every signing must be accompanied by payment to one particular agent who "always raises his head".

A common view is that transfers are the result of pushy agents unsettling players in order to profit themselves, but I hadn't realised they were motivated as much by fear of losing their player as by cashing in. "In your two-year period, you've either to get him a new deal or a move, regardless whether he's played really well. We say 'if you're working hard, we're working hard', but some of them don't see it that way."

They must demonstrate their worth or risk not being re-signed. "You're vulnerable every two years. That's the truth of it. So as you get to 18 months in you're already thinking 'how's he going to react when you say 'we need to re-sign'." The Association of Football Agents (AFA) is in favour of extending that period to four years so that, in theory, agents would stop engineering moves and be less eager to please.

Colleagues are viewed suspiciously and rivals are a constant threat. Allowing someone unsupervised access to their players can leave an agent vulnerable, so even hiring new staff is problematic. A player can look to be released from an agreement at any time if a middle man convinces them it's the right thing to do. Most agents would have experienced this and they all respond the same. "If you're going to terminate this early, I want some money from you or the agency you're going to go with." But unless there's documented proof of, say, financial misdemeanours, players cannot easily walk from their agent.

The pressure to keep players happy is as fierce as the competition to sign them in the first place. Agencies have people at schoolboy games handing out business cards to parents. "Hookers" and "goafers" are deployed at nightclubs to befriend players in the hope of earning their trust. The recent experience of a 14-year-old was cited to me to illustrate how social media sites are also being used to make contact. "It's happening with kids. It's grooming. It's a free-for-all. These kids are seen as lottery tickets."

I was a full international by the time I was offered the £50,000 back in 2002, but a 17-year-old at a Premier League club was recently paid £100,000 to sign with an agency despite not yet making a competitive first-team appearance.

In relation to Irish youngsters, the agents repeated a view I have heard many times in terms of the physical development of players here. "If you took an under 17 from Ireland, he'd be the equivalent of an under 16 and he'd need a year to acclimatise and to catch up," said one of the agents. It's why, they both believe, fewer players are travelling to England at the younger ages.

I had five different representatives in a seven-year career and my first job post-retirement was with a football agency. I thought I had a reasonable understanding of how football worked but I was surprised by the corruption these agents alleged last week was taking place. I was once told that for an agent to be corrupt an ally on the other side of the table is needed. It seems there is no shortage of people willing to provide that role in football today. "There's so much money involved. And they're such devious bastards, as I discovered when we rocked up to [Premier League club] with [player]."

Towards the end of our conversation I asked what would happen if they spoke publicly on the record about what we had just discussed. Calling it "sheer cowardly pragmatism," their response was unequivocal: "No one would deal with us in those circumstances."

"But just to get the balance here, I wouldn't swap it for anything else. I compare it to some of the other things I have done. It's so much better, more interesting, more rewarding, on every level. It's only a selling game, remember, and any salesman will tell you the moment the guy says 'yes' and you get back in your car, you are buzzing. Or the call comes from the Premier League club to the boy that's in League One. It's a high like very few other highs"

Those at the top can make money like few can make money, but the majority know that the flipside of the highs is a career of neverending uncertainty.

Sunday Independent