They are part of a 32-man squad which will be led by the coaching team of Steve Bates, Jim Mallinder, Mark Hopley, James Scaysbrook and Richard Whiffin.
Both tighthead prop Heyes and fly-half/centre Hardwick were capped at Under-20s level last season and have enjoyed spells of first-team rugby with Tigers this year.
They are among 14 of the squad already capped at this level and the 10 who played in last year's final of the World Rugby U20 Championship in France.
England Under-20s open their Six Nations campaign against Ireland in Cork on February 1 and then host France in Exeter before a trip to Wales. The second home fixture is against Italy at Bedford and final round is against Scotland on Friday, March 15 at Northampton Saints.
The World Rugby Under-20s Championship will be held in May/June.
The squad meets at Bisham Abbey this Sunday (January 6) for a week-long training camp before coming together again at the end of the month ahead of the Six Nations.
Steve Bates said: “Selecting the 32-player squad was a very difficult task which reflects the quality of the group. We have several players with Premiership experience and that speaks volumes of how good they are.
“The expectation for us now is to raise the standard and quality of performance from last season’s group and for the players to continue to develop in this environment.
“We used close to 40 players in last year’s Six Nations, there will always be injuries and those outside of the 32 will still be required so my message to them is to stay fit, stay interested and to continue playing well.”
Forwards: Ollie Adkins (Gloucester), Josh Basham (Durham University), Will Capon (Bristol Bears), Richard Capstick (Exeter Chiefs), Alex Coles (Northampton Saints), Nic Dolly (Sale Sharks), James Dun (Bristol Bears), Joe Heyes (Leicester Tigers), Ted Hill (Worcester Warriors), Aaron Hinkley (Gloucester), Joel Kpoku (Saracens), Samson Ma'asi (Northampton Saints), Kai Owen (Worcester Warriors), James Scott (Worcester Warriors), Marcus Street (Exeter Chiefs), Tom Willis (Wasps).
Backs: Tom De Glanville (Bath), Fraser Dingwall (Northampton Saints), Connor Doherty (Sale Sharks), Ollie Fox (Yorkshire Carnegie), Tom Hardwick (Leicester Tigers), Ollie Hassell-Collins (London Irish), Josh Hodge (Newcastle Falcons), Ollie Lawrence (Worcester Warriors), Sam Maunder (Exeter Chiefs), Cadan Murley (Harlequins), Cameron Redpath (Sale Sharks), Arron Reed (Sale Sharks), Ollie Sleightholme (Northampton Saints), Marcus Smith (Harlequins), Gus Warr (Sale Sharks), Kieran Wilkinson (Sale Sharks).