I don't see anything in API-650 that would prohibit or allow that. You might check your weld procedures and see if they incorporate provision for that.

Please advise the shell plate thickness, grade and service. This is a lot different if the shell is 1/4" thick or 1 1/4" thick, plain carbon steel or duplex stainless, plain tank or elevated temperature or pressure, etc. You may get a lot of shrinkage that will tend to warp or flatten out the plate which you will have to control or risk being out of dimensional tolerance(s). As JStephen mentioned you'll need a weld procedure that includes such a gap. You might want to perform additional NDE to assure full fusion and no hard spots. And get buy-in from the owner.

If this is the final course of shell plates, you must have other vertical joints in this courseThese other vertical joints should not be welded yet, right. If not, refit the other vertical joints and divide up this 1/2" gap to the other joints.For instance, if you have 4 vertical joints that are not welded, refit them increasing their gap by 1/8". 4 x 1/8=1/2"
In other words spread the 1/2" gap among the other plates equally.

I think the solution @matschka has proposed is much better.

I am wondering what if the rest of the plates have already been welded?

Clause 5.1.3.8 of API 650 says, "Permanent weld joint backing strips are permitted only with the
approval of the Purchaser".
Can someone interpret this clause for me? Because what I understood is that only permanent welded backing strips are not allowed without client's approval, those which shall be removed afterwards have no issue with the code, except for the post weld HAZ's and dimensional tolerances.

Regards,
Asad Ali

Generally in tank construction,a course of plates are erected. These can be the 1st ring of plates or the 2nd, 3rd, 4th. etc. When erecting the plates they are temporally 'pinned'
together with a 'keyplate'. These are used to hold the plates together and will be used to obtain the fit up 'gap. Usually 4 or more keyplates per plate are used. The plate erection will continue until all plates required in the course are temporarily fixed in place. Then, final fit-up will start. No welding can occur before all the vertical plates are fit-up. The key plates will be used to obtain the required root gap. Once the required 'root gap' is acceptable,the weld joint will be secured by welding 'finger bars' (2"x6"x3/8"cs) across or transverse to the vertical joint. Several finger bars will be used per weld joint. These are used in leiu of tack welding the joints. All the vertical joints will be fit-up before welding can start. This is because you might have to big of a root gap on the last 'vert'. Accordingly, you will have go back and adjust the other fit-ups and divide the large root gap among the other fit-up joints. Good Luck
The other question refers to leaving or removing backing strips from the shells of tanks.
Most tanks are welded with the open butt technique. If backing strips are used and left in place they would look "terrible'. Its a question of cosmetics.

Hopefully, they have this sorted out in the 5 months since it was posted.