It's a weed! and a perennial one! Botanical name is Heracleum sphondylium - also known as cow parsnip, hogweed and (in Suffolk) cow mumbles. It will have a white flowers at the top of a tall stem.
At this time of the year, as it's in a border, it may be best to just keep cutting it to ground level (the leaves can be composted) and then dig out the root in the Autumn/Winter - it has a deep root and any piece left in will grow again.