The 25-year-old Belgium international still has two years remaining on his existing deal at the Stadium if Light, but agent Nico Vaesen has revealed the prospect of him signing an extension is remote with talks to date unproductive.
Vaesen told the Sunderland Echo: "There is no progress. I think it is very unlikely that he will sign a new contract.
"Simon has obviously come to a point where he wants to step up a level and there is a lot of interest in him.
"He is very ambitious and wants to play in the World Cup in Brazil next year.
"It was a good thing that Sunderland stayed up and it is a fantastic club where he has had three great years.
"But it is maybe time to step up a level and play for a club competing for a European spot."
The news will come as a huge blow to manager Paolo Di Canio, who may now have to decide whether or not to cash in on one of his prized assets either this summer or in 12 months' time if he is to avoid losing him for nothing when his contract does expire.
Mignolet was arguably Sunderland's most accomplished performer during a difficult last Barclays Premier League campaign, and has since been linked with a big money move away from Wearside with Arsenal and Liverpool among the clubs touted as possible destinations.
He is currently behind Chelsea's Thibaut Courtois, who spent last season on loan at Atletico Madrid, in the Belgian pecking order, but has high hopes of forcing his way into the national side.
Mignolet joined the Black Cats from St-Truiden in June 2010 for £2million and the club would be sure to make a healthy profit on him if they were to sell.
However, Sunderland are desperate to retain the services of a man who has proved hugely dependable in recent seasons and it remains to be seen how doggedly they will fight for him.
Vaesen said: "All teams that are interested in Simon, we direct them to Sunderland.
"He still has a two-year contract there and any interested clubs have to agree a price with Sunderland first. That is the way we conduct our business."
The news comes just days after Mignolet's under-study Keiren Westwood revealed he may have to consider leaving the club to play regular football in a bid to regain his place as the Republic of Ireland's number one.