What can I do with a white poplar that cannot hold itself upright?

Not sure, it was obviously not a very strong stemmed plant.
Could stake it high up losely so as not to hold it upright, just give it the idea of what is should be doing.

It probably started because it was 4 foot tall and staked 3 foot up meant that the tree didn't have to rely on the wind blowing its top around allowing it to naturally strenghen its trunk to cope. If that makes sense.
Normally it should be staked a third the way up so the root ball stays firm, when top is waving around.

Thanks Nicky - I think it could have been staked too high but that doesn't help now unfortunately. I was wondering whether to lop the top off, to give it a better chance?