Our native species of bluebell (Hyacinthoides non-scripta) is threatened by the spread of Spanish squill (Hyacinthoides hispanica), a similar species imported into our gardens from southern Europe during the 17th and 18th centuries.
Our native bluebells are dark blue and fragrant, with flowers on just one side of the stem producing that characteristic droop; the stem and the leaves are slim and flexible. Squills’ flowers are lighter in colour, often striped, and can be white or pink as well as blue. The whole plant is more upright; the stem and leaves more robust.
Bluebell 
Spanish squill
The two species are genetically so similar that they cross pollinate, producing a hybrid. Hybrids are usually weaker, less successful than the parent-species, and infertile – but not in this case. The Hyacinthoides hybrid is tough, adaptable and very fertile; it produces more seed than either bluebell or squill and that seed germinates more easily and in a much wider range of conditions.
The Woodland Trust has estimated that one in every six of our bluebell woods has been invaded by Spanish squill and the resulting hybrids are out-competing the bluebells. Climate change is already threatening the delicately balanced environment in which bluebells thrive; the pressure of an alien competitor for that changing environment could be very damaging.
Ian, a Friend of SCP, found Spanish squill among the bluebells in the copse at the bottom of Sheep Field several years ago. Each spring since then, he has conscientiously pulled out or dug up the squill flowers to stop them seeding or cross pollinating. Last year he reported that there were no squill flowers in the copse, though this does not necessarily mean there are no squill bulbs below ground. This year, in lockdown, it will be more difficult to check, but check we must.
We have to be more careful about moving species around the world for our own purposes.
Header photograph: DKG
Other photographs: Google Images
More about alien invasions:
Southwick Country Park Nature Reserve 
We do have several squill bulbs among the bluebells in our garden planted by the previous owners many years ago. Is that why we also have clumps of white bluebells and in the past a clump of pink bluebells too.
Not necessarily. Native bluebells can throw up the odd pink or blue sport without any hybridisation but if you have squill, then some of your bluebells are almost bound to be hybrids. Any hybridisation does seem to result firstly in the flaring out of the flower shape but it can be hard to tell.