Celtic's resolve to hold on to Fraser Forster is set to be tested by Sunderland after the agent who helped bring Simon Mignolet to Wearside from Belgium endorsed suspicions that the goalkeeper is keen to leave the Stadium of Light.

Sunderland's outstanding performer last season, Mignolet has two years left on his current contract but is admired by Arsenal and Liverpool and, despite Paolo Di Canio's determination to retain him, could move on this summer.

Nico Vaesen, whose claims to represent the 25-year-old were disputed by Guy Vandersmissen who maintains he is the Belgium international's official agent, said that talks about a new deal have been unproductive. "There is no progress," said Vaesen. "I think it is very unlikely that Simon 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 is maybe time to step up a level and play for a club competing for a European spot."

Although Vandersmissen's stance is slighty softer, he, too, gave no indication Mignolet would be agreeing another contract. "I am the representative of Simon Mignolet," he said. "Simon is my player, I've been working with Simon since he was 16 years old. At this moment there are some clubs who are interested but Simon still has a two-year contract in Sunderland, so we depend on the position that Sunderland is taking in relationship to an eventual transfer of Simon Mignolet. But I don't think at this moment we would be prepared to sign a new contract."

It all leaves Di Canio – who has made a £7.5m bid for Livorno's 27-year-old Brazilian striker Paulinho – with a dilemma. Does Sunderland's manager cash in on Mignolet and force through a move for Forster – the Celtic keeper is understood to believe his chances of establishing himself in the England squad will be enhanced if he moves to the Premier League – sell the possibly disgruntled Belgian next summer or lose him for nothing in 2015?

Mignolet – bought from Sint-Truiden for £2m by Steve Bruce three years ago – has fallen behind Chelsea's Thibaut Courtois, who spent last season on loan at Atlético Madrid, in Belgium's first XI and is anxious to establish himself as his country's first choice ahead of next summer's World Cup in Brazil.

Meanwhile Keiren Westwood, Mignolet's understudy, has said he is considering leaving Sunderland to play regular first-team football as he attempts to recover his place in the Republic of Ireland's goal.