Khan had been promoted by Warren since turning professional in 2005
Promoter Frank Warren has spoken of his disappointment at Amir Khan's decision to leave his camp and join American-based Golden Boy Promotions.
Khan, 23, left Warren after five years having been guided by the promoter since turning professional in 2005.
"I do not mind admitting I was gutted at the way Amir Khan and his team brought our partnership to an end," Warren wrote in his column for The Sun.
"These days it seems loyalty is a quality bestowed in few sportsmen."
Under Warren's guidance Khan captured the WBA light-welterweight world title, winning 22 of his 23 fights.
But Khan announced on Sunday that he would be joining promoters Golden Boy, who are led by Oscar de la Hoya, as the Bolton fighter looks to crack the United States, allowing him to increase his global profile.
Khan has been based in Los Angeles since linking up with revered trainer Freddie Roach in 2008, following his humiliating first-round defeat by Breidis Prescott.
Roach also trains five-weight world champion Manny Pacquiao, who is recognised as the world's best pound-for-pound boxer.
The 2004 Athens Olympic silver medallist has a 'tentative agreement' in place to face Argentine Marcos Maidana in Las Vegas on 14 April as looks to make his American debut.
Warren, who guided Ricky Hatton and Joe Calzaghe to world titles, revealed his regret that it was Khan's father Shah who broke the news to the promoter, but only after he had found out from another source.
"I told him (Shah) I was offended he should say Golden Boy Promotions could do a better job than me and annoyed they had chosen to tell me 48 hours after I had heard the news elsewhere," said Warren.
He is very talented, great for boxing and I genuinely hope he does well
"It is amazing to think that the last time Amir and I were together was in Newcastle - after his demolition of Dmitriy Salita - when he told the Sky TV cameras: 'Me and Frank are going to America'.
"He repeated that statement in the post-fight press conference.
"Yet, just a few weeks later, I was left to hear it 10th-hand that our partnership was over."
Warren added: "Under my promotional banner, Amir became light-welterweight world champion and banked millions of pounds.
"And I do not even hear of the split from the horse's mouth, after all I have done for him.
"Remember, just four fights ago Khan was wiped out in less then 60 seconds by Breidis Prescott.
"Despite the way the Khans have handled this, I did my best for Amir and it is disappointing he has not even picked up the phone.
"But he is very talented, great for boxing and I genuinely hope he does well."
"Frank is a great promoter, and we've had a great career together," Khan told the BBC in response.
"I wanted to go with a promoter in America, and I hope they can take me to the next level. I wanted to fight over there and make my debut in America and then bring the big fights back here."