English translation: This factor is essential to the development...
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Explanation: I think it's more of a guarantee than a contribution, but I would reword it, as above, or: "This factor is an essential part of the development". (I should think "the" should probably be included but can't be sure without more context).
Thanks for you help. This was a tricky one as is evident from all the various replies to my question. I am in the fortunate position of being able to flag up ambiguity with my client and so this is what I have done in this case. 4 KudoZ points were awarded for this answer