Hamilton and Max Verstappen came to blows on the first lap at Silverstone on the run down to Copse.
Hamilton attempted to go down the inside of his title rival into the high-speed right-hander but his front tyres hit Verstappen’s rear, sending him into the barrier.
The seven-time champion picked up damage but the red flag stoppage allowed Mercedes to fix his car.
The stewards handed Hamilton a 10-second time penalty for his role in the incident but he still went on to win the British Grand Prix for a record eighth time.
After the race, Horner told Sky Sports: “It shouldn’t be like that, to be honest with you. Max has incurred a 51G accident. Lewis Hamilton is a seven-time world champion, he shouldn’t be making manoeuvres like that. It’s unacceptable. Thank goodness the biggest result for us today is that he’s uninjured. He’s having to go to hospital for precautionary checks after a 51G accident. I hope Lewis is very happy with himself.”
Giving his view on the incident itself, Horner explained: “He stuck a wheel up the inside in a corner that you just know you don’t do that. You just don’t stick a wheel up the inside at Copse, in that corner, in that circumstance. He was nowhere near ahead, it was contact, left-front to right-rear.
“The speed that they’re travelling is one of the fastest corners in the championship. Lewis has got more than enough experience to know that that’s unacceptable. As I say, I am just very disappointed that a driver of his calibre should make such a move like that. It’s dangerous. It looked desperate. Thank goodness uninjured but he’s put a competitor in hospital getting himself checked out.”
The interviewer then tried to give Hamilton’s view of the incident, with Horner replying: “It wasn’t much of a penalty was it really? I don’t care what Lewis said. Have a look at your own analysis, draw your own comparisons. For me, that’s a hollow victory.”
Verstappen is currently in hospital for precautionary checks.