Millwall fans are furious he is quitting the Den less than six months after signing a new £7,000 a week deal - and just days after having a transfer request rejected by the south London club.
But Morison insists he had a verbal agreement with Millwall that he could leave if a Premier League club came in for him.
And he also claims the Lions asked him to put in a transfer request "to get the process going".
He told The Sun: "When I signed the contract in January the agreement with the club was that I would stay until the end of the season - and I did that.
"Had I wanted to, I could have kicked up a fuss and left then because there offers on the table.
"But I stayed and signed the new contract, like the club wanted me to do, on the understanding that if a Premier League club came in for me, we would have a look at it.
"There was nothing written in my contract but I had conversations with the club and they knew my position. They would have been naive not to.
"They were well aware of my ambitions.
"I told them if we didn't get into the Premier League ourselves and the opportunity came up this summer for me to do so, it was an opportunity I wanted to take.
"Now a Premier League club have come in for me and I feel it is the right time to go. New contract or no new contract."