I'm confused: as far as I know, Bitcoin Core doesn't offer a 12 word backup seed phrase. I haven't made a fresh install for a while, but I'm pretty sure this hasn't changed.
Where did you download Bitcoin Core, and are you sure your computer is clean?
Ok so I downloaded it and imported BTC address, yet its only read only ( I can see BTC balance).
Importing your address indeed produces a watch-only wallet. You'll need to import the seed phrase to be able to spend it, but I would recommend to wait a bit until it's clear where the seed phrase came from. And make sure you only download the real Electrum, there are phishing sites around.