Is it legal to poison Otters in the UK?

I own a farm near Corsley Heath in Wiltshire. Recently a family (or two for all I know) of Otter's has moved into the river that borders my land. Unfortunately these Otter's have been stealing my crops and it is going to impact on my profits come harvest time. I have it in my mind to shoot or poison these vermin but am worried as they are the 'flavour of the month' amongst the middle classes if this is legal?