Yes, IELTS is the best bet to avoid any issue later. Without IELTS, point allocation depends on VO; which is not the case when results of IELTS are submitted.
Honolulu said:If you are not a native speaker an IELTS score is your safest bet.
Otherwise a good documentation, preferably with proper evidences should work for your language requirement.
My thought is that they wont send it back , but will ask for the documentation.
But again, depemds on what chances you wish to take with this process...
I would play safe rather than sorry.