Gallagher has been linked with a move away from Stamford Bridge for some time, largely due to his status as an academy graduate.
The midfielder's sale would generate pure profit and help Chelsea avoid falling foul of the Premier League's Profit and Sustainability Rules.
Having entered the final 12 months of his contract, Gallagher has seen speculation surrounding his future grow bigger and bigger.
Chelsea have been open to extending the player's contract as they look to avoid the risk of losing him on a free transfer next summer.
Gallagher rejects Chelsea contract as Atletico make their move
However, according to The Athletic, Gallagher has opted to reject the chance to sign a new contract with Enzo Maresca's side.
The report claims that Chelsea offered the 24-year-old a fresh two-year deal with the option to extend by a further 12 months.
The deal would have put Gallagher on par with the highest earners in the Chelsea midfield, but he has ultimately opted to reject the proposal after he reportedly turned down another contract offer in June.
The news of the player's contract decision comes as talkSPORT report that Atletico have reached an agreement with Chelsea to sign Gallagher for £33m plus add-ons.
It is also believed the Blues accepted an offer from Villa earlier this summer, but the Chelsea academy product rejected the chance to join Unai Emery's side on a three-year deal.
Atletico wait for Gallagher decision
While Atletico may have an agreement in place with Chelsea, it is still currently unclear whether the Englishman will swap Stamford Bridge for the Civitas Metropolitano.
The Spanish club are yet to agree personal terms and are now waiting to hear whether Gallagher is open to the transfer.
Gallagher could follow in the footsteps of his England teammate Kieran Trippier, who became Atletico's first English player in 95 years when he joined Diego Simeone's side in 2019.
The right-back spent around two-and-a-half years with Atletico, helping the club win the 2019-20 La Liga title before he left for Newcastle United in January 2022.
Atletico seemingly want to add Gallagher to their current central midfield options which include Rodrigo de Paul, Marcos Llorente, Arthur Vermeeren, Pablo Barrios and Koke.