Flores, 25, has been in a contract negotiations with Adelaide since the end of his stunning season despite still having a year left on his existing contract.

The Argentinean spoke out about the speculation earlier this month to insist he wanted to stay at Adelaide, but hoped they would match what he could get at other A-League sides.

He said then he would not be comparing Adelaide's contract offer with what he could get overseas.

Now though he is said to have spurned the Reds' best offer of $320,000 a season to take $800,000 a year at the CSL side.

The deal is expected to go through as soon as the international transfer window opens on July 1, and could play for his new side next Saturday if the paperwork is rubberstamped in time.

"We were told on Friday afternoon by Marcos," Adelaide United chairman Greg Griffin told the Adelaide Advertiser.

"We tried to dissuade him, we are very disappointed."

The club is facing a furious backlash from fans over the deal, with many venting their anger on Adelaide's Facebook page within minutes of the news breaking.

"You've just sold the best thing that's ever happened to Adelaide United, and for what?? 500k?? Burns and Djite went for more than that. And Flores is 10x the player either of them two were," said one fan, Shane Robinson.

"Marcos Flores was bringing in AT LEAST an extra 2-3000 fans a game last season. So say goodbye to memberships. Say goodbye to average 11,000 crowds. Say goodbye to happy fans."

Another, Dimi Sianis, sent a furious message to the management: "Angry, sad, upset, raging....I'm more than that.

"You really don't understand what the consequences of this are going to be, do you?"