The Croatia midfielder exclusively revealed in Saturday's Sportsmail that he wished to join Chelsea, but Tottenham boss Redknapp has urged the club to convince Modric he can fulfil his ambitions at White Hart Lane.
Tottenham failed to qualify for the Champions League after reaching the quarter-finals last season and Modric has insisted he wanted to sign for a 'bigger club', play in Europe's premier club competition and win titles.
Wanted man: Luka Modric is ready to turn his back on Tottenham
'If Tottenham are looking to go anywhere, we can't be selling Luka.
'We have to do something to keep him at the club. We can't afford to sell Luka Modric if we want to be a successful club.
'We have to show him our ambition, which we will do. If we can add one or two good players to our squad there's no reason we can't have another great season.
Tottenham could be a great club for him to stay with for the next few years. That's what we're hoping for.
'He's a fantastic lad. You'll never meet a better lad than Luka. I suppose he's seen a massive contract and the chance to win the Premier League and play in the Champions League and that's what he wanted.
'But we want that as well. We want to get back in the Champions League and we need him to help us do that,' the Spurs boss told BBC Radio 5 Live's Sportsweek.
Steely determination: Harry Redknapp (left) and Daniel Levy have made their feelings about Modric known
Jack the bad. 'Don't see all the fuss about Modric wanting a move . he has done well at Spurs but he needs to play Champions League!' - Arsenal's Jack Wilshere set himself up for a hail of abuse on Twitter when he backed the Croat to quit Spurs yesterday and later told his 'haters' they couldn't 'phase' (sic) him.
Daniel Levy issued a strongly worded statement on Saturday saying Modric was not for sale 'at any price' and that none of his key players will be sold this summer.
'We are building a team for the future to consistently play at the highest level, and retaining quality players is crucial to that,' he added.
Redknapp said it was vital the chairman kept his word. 'If Daniel's saying 'no', he can't be seen to be going back on that now. He's made a very strong statement. I wouldn't see Daniel backtracking on that at any price.'
The Spurs boss also said Chelsea are not alone in their interest in Modric, for whom they have made an initial ?22million bid. Manchester City and Manchester United have also been tracking the midfielder, but the 25-year-old is keen to stay in London.
Not for sale: Modric (right) is not heading to Chelsea, according to Tottenham's chairman
A Spurs source said the club are 'monitoring' Chelsea's public pursuit of Modric, having made their stance 'abundantly clear' in writing. But Sportsmail understands an official complaint had not been made to the Premier League this weekend.
Redknapp admitted Chelsea's interest in the midfielder was 'difficult to understand', when the west London club have yet to appoint Carlo Ancelotti's successor after sacking the Italian on the last day of the season.
The Tottenham boss said: 'Chelsea are not the only club that have been in for him. There have been other clubs top clubs. But (the Chelsea interest) is very strange and difficult to understand.
'This has been going on for quite a while, not just this week.It's been going on for the last three or four weeks, since Carlo Ancelotti lost his job. Somebody else at Chelsea is choosing the players not the new manager.'
Playing at the highest level: Modric dazzled during Spurs's champions League campaign
Redknapp also played down the suggestion striker Jermain Defoe could leave White Hart Lane, potentially for arch-rivals Arsenal.
The 28-year-old striker told Sportsmail last season he had considered his future after failing to secure a regular place in the Spurs side, but Redknapp rejected the idea, saying: 'I didn't see that. Icouldn't see that one happening.'
Harry Redknapp: Spurs will never sell Luka Modric to Chelsea - or any other clubEXCLUSIVE: Let me leave! Modric bombshell as he admits he wants to join ChelseaTottenham chairman Levy: Modric is not for sale 'at any price'All the latest Tottenham news, features and opinionAll the latest Chelsea news, features and opinion
Explore more:People: Daniel Levy, Harry Redknapp, Luka Modric, Jack Wilshere, Carlo Ancelotti Places: Manchester, London, Croatia