Should we cut off this sucker?

We have been clearing parts of our land which were covered by bramble and rubbish left by the previous owner. We've uncovered the base of a rowan (we think) tree and found a very mature sucker growing from it (photos below). Question: should we cut off the sucker and if so, is this a good time of the year to do it?
We don't care about the ultimate shape of the tree, we just want to maximise its chance of survival as it seems to have suffered from the brambles. The sucker might also be causing the tree to lean. We're just not sure if it would weaken the tree further or put it at risk of disease if we were to cut off such a mature branch.
Any thoughts/advice would be appreciated. Thank you.

 

Suckers grow from the roots of the tree and can appear anywhere along the root run.
Yours looks like a branch that's very low down.
So yes you can cut it off with a pruning saw to leave a stub on the trunk that is about 1" long.
Ideally done when the tree is dormant, but they're tough trees and it's unlikely to suffer if you do it now.
Or you can leave it.

I have one that I cut down to about 2ft every few years and it sends up lots of new shoots each year.
When it all gets too tall again, I cut it all back to 2ft.

 

It's most likely leaning towards the light, but you can prune off the low branch if you like.

 

Doesn't look like a sucker as such, just a low branch.

 

Thank you both for your help. As it's just a branch, then there's no urgency to cut it off and we'll see how the tree does over the next few weeks.