There has been controversy in recent years over the lack of involvement by British suppliers in offshore projects versus the amount of government investment.
When Vestas completed the 300MW Thanet offshore wind farm in September 2010, it was revealed only 20% of the construction jobs to build it were UK-based. The 1GW London Array scheme was even worse, with reports suggesting UK companies only built 10% of it.
The union is asking E.on to indicate how many jobs will be created, in manufacturing, building and operating the project. It is also seeking assurances on how the project will benefit the local economy and that of the UK as a whole.
If consent is granted by June 2014, onshore work should start in 2015, with offshore construction beginning the following year. Rampion should be fully operational by 2018-19.
The GMB is concerned that the wind farm destined for the Brighton seaside will bring no benefit to the south coast economy.
GMB national secretary for energy, Gary Smith, said "The GMB wants the facts from E.on as we suspect that the turbines will be sourced in Germany or Denmark. Where is the up side for the UK economy if the turbines are made on the continent, brought over on Dutch barges and connected with cables manufactured in China?"