"TNT scum! How dare you set foot in the Post Office? Get the fuck out!"

This was the hostile introduction I got on my first day working undercover as a TNT postman for Channel 4's Dispatches investigation, Secrets of Your Missing Mail.

A weathered old-time postman was visibly irate when he saw me in the distinctive orange uniform of Royal Mail's rivals TNT, venturing on to his patch as I delivered a letter to a Central London Post Office . While this postie had picked the wrong person to do battle with – after all, I was just another guy trying to do a job – I soon understood his angry reaction to the imminent privatisation of the Royal Mail. Apart from the obvious threat to universal service, privatisation represents a direct affront to the working conditions that have been so hard-fought for by workers and unions over the years.

Last week marked the formal announcement by Michael Fallon, the minister for business and enterprise, that the Royal Mail will be sold off by next April, setting the ball rolling on what is set to be the biggest privatisation for over 20 years. This follows the deregulation of postal services in 2006, which allowed companies like TNT Post to win contracts to deliver mail from the supplier all the way to the letterbox on behalf of private and public sector organisations. TNT Post, who I worked for over the course of a month, are in a pilot phase in West and Central London this year, providing competition to deliver letters directly to the doorstep for the first time in Royal Mail's 360-year history. If successful, TNT will expand its operation across other parts of the country in the next five years, aiming to employ up to 20,000 postal workers.

So what does this mean for the 134,000 postal workers represented by the Communication Workers Union? And what can these changes in the postal industry teach us about how ongoing privatisations affect workers? As postal worker and blogger Roy Mayall points out, private companies are allowed to bid for these contracts with no obligation to meet the pay and conditions that Royal Mail workers have fought for over the years. During the time I spent as a TNT postman, I was able to see firsthand the ways in which widespread privatisations are leading to regressions in working conditions, with private companies exploiting the large numbers of desperate young unemployed to offer employment packages far inferior to their Royal Mail counterparts.

I was one of a growing number of workers on what is known as a "zero-hour contract". This term is used to describe an extremely precarious form of contract, in which workers are not guaranteed any hours of work. The number of major employers hiring on zero-hour contracts has risen from 11% in 2004 to 23% in 2011, with unions blaming government privatisation of services for this rise, denouncing these contracts as a throwback to the Victorian era.

Denied any fixed hours of employment, I was forced to hustle for my next day's work on an almost daily basis. Sometimes I took a gamble to come in as a "relief worker", arriving at the depot at 7.30am in the hope that someone would have dropped out so I could cover their rounds.

While zero-hour contracts prove convenient for students and part-timers, the reality is that these groups make up the minority of the workforce. For most workers, it proved to be a myth that zero-hour contracts translate to greater flexibility; I was advised by several colleagues that I should be ready and available for work whenever called upon if I hoped to get more regular shifts. It became clear that the balance of power in these arrangements is heavily weighted in the favour of employers.

Indeed, bosses are inclined to over-hire staff to ensure that they will always have enough staff for any given shift, which can leave workers without enough hours to make a sufficient living: on £7.10p/h (with London weighting), it is nearly impossible to make ends meet if you miss out on shifts for just a day or two. Workers are left walking this financial tightrope on a week-by-week basis.

This kind of "underemployment" is particularly insidious considering the fact that many of those recruited come off benefits upon gaining employment, only to earn an insufficient salary to adequately live on once they are actually in work. Official statistics of course show these people as being employed, despite the fact that in reality they are denied regular hours. This leaves workers obliged to navigate an uncomfortable no-man's land between secure work and benefits.

Furthermore, these kinds of contracts leave workers almost entirely at the whim of their bosses. With the decision as to who gets given shifts ultimately resting with the supervisor, workers are left vulnerable to favouritism. Employers do not need to find a reason for dismissal; they can simply phase out shifts until employees find themselves with little or no work.

A growing private sector workforce is being forced to live with no guaranteed level of earnings, unpredictable schedules, weak employment rights and precarious conditions.

In light of this, I can't say I blame the Royal Mail postman for losing his rag with me.