Mainwaring-Smart, 29, suffered a broken back, collapsed lung, cracked eye socket, broken wrist and broken ribs after colliding with Filip Backlund in race one at the Kent circuit on Saturday.
The Anvil Hire Yamaha rider, who tweeted a picture of himself in hospital after the crash, was taken to King's College Hospital for examination where the full extent of his injuries were revealed.
A track official was also taken to hospital after being struck by debris from the crash.
British Superbikes title winner Shane Byrne, a friend of Mainwaring-Smart’s, said: “I believe he had a problem with his bike at the back straight - that’s about 170mph.
“He had a problem with his brakes, went straight and took a massive knock.
“We’ve been texting a bit but that picture really is a reminder about how dangerous it can be.”
Now based in Kent, Mainwaring-Smart was making his comeback after sustaining a broken collarbone at Cadwell Park earlier in the season.
Meanwhile, Jenny Tinmouth’s British Superbikes season ended with disappointment after a crash during qualifying saw her miss race day at Brands Hatch.
The Ellesmere Port rider recorded her fastest ever lap at the Kent circuit during the first qualifying round of the season finale but suffered a big crash shortly afterwards.
Tinmouth, 38, emerged unscathed from the accident and was given the all clear by medics following a precautionary trip to hospital but failed to return for the main event owing to damage to her Fireblade.
Backford’s Joe Francis rounded off his British Supersport campaign with a third-placed finish in the feature race at Brands Hatch.
The 18-year-old finished in seventh spot in the final standings.