tell them you love them but you dont get that spark from them anymore and if they are honest they will agree that something is missing wether its from your side or theirs and if they take it badly and get a bit nasty or unreasonable you have even more evidence to use that it doesnt click but whatever you do do it soon then give me a call ;-) lol good luck
be completely honest with them as beating around the bush makes things harder to say. if it needs to be said then just say it. there will be emotions going wild but that is to expected. let them cool down for a few days then check to see if they are ok and try to agree that you can still be friends x