The Power Maxed Vauxhall driver was firstly deemed to have got an advantage after nudging into the back of leader Chris Smiley at the Wilson hairpin on the opening lap of the race.
He was punished with a five-second censure for that incident, and then hit with a further 10-second penalty following contact with Ash Sutton’s BMR Racing Subaru Levorg while the pair were disputing the lead.
Their duel erupted on lap nine of 12 and the pair, who were teammates at Team BMR in 2017-18, touched several times. There was contact at Agostini as Plato regained the lead.
The final straw came as eventual winner Rory Butcher jumped them both on the Bentley Straight and Plato made contact with Sutton going through the Brundle left-hander.
Plato was able to continue to finish in fifth, while Sutton came home in 20th position.
Sutton was fuming afterwards, saying: “It was brainless. It is just stupidity. [He had hit me several times] and then [Plato] gets his joker card out, and he turns me around under the bridge. That was the third time he had hit me.
"He is absolutely brainless, and we will take that [to the stewards].”
Plato said he didn’t think he had done anything wrong during the race and was furious about the contact on track.
“Ash is pissed off? He needs to grow a pair of balls,” said Plato. “He needs to look back at the videos and see how he drives sometimes. I don’t believe I initiated the contact with him.”
There was a further penalty for Speedworks Toyota driver Tom Ingram, who finished eighth. He was deemed to have gained an unfair advantage as he passed the Motorbase Ford of Ollie Jackson, who finished ninth.
The clerk issued a 6.7s time penalty to Ingram, which pushed him back behind Jackson in the finishing order.