Mums have collected nearly 200 signatures for their petition demanding creche facilities at the new Streatham leisure centre.
The gym and swimming pool closed in 2011 following Lambeth Council's £26m deal with Tesco to jointly fund a new leisure centre, alongside a Tesco superstore.
Mother-of-two, Catherine Thomas, who started the petition, said: "Wandsworth, Tooting and Brixton leisure centres all have a creche. Having spent millions on the new complex, we don't understand why a creche will not be provided in Streatham."
Mrs Thomas, who has a two-year-old daughter and six-month-old baby, said: "I will not be able to go to the gym or take my older child swimming without creche facilities for my baby."
The petition is backed by Streatham Mums Network, a group with over 1,300 members; reflecting the large demographic of parents with young children in Streatham.
Zoe Dieckhoff, a Streatham resident expecting a baby, said: "It's a real blow to find out that there won't be creche facilities available.
"I live behind the new centre and during it's long period of construction I have been picturing myself making regular use of it for swimming with my baby and later on for getting back into shape - for which the creche would be essential."
Chantalle Moverley, who has a sixteen-month-old baby and another one on the way, is also disappointed she won't be able to use the gym after giving birth.
She said: "People who are around during the day are young mothers. The council are offering concessions for day-members, but without a creche we won't be able to use the facilities."
Councillor Sally Prentice, cabinet member for culture and leisure, said Lambeth Council is looking into the matter.
She said: "In principal it is something which I believe is important. We want to encourage as many people as possible to use the leisure centre, including parents with young children."
Streatham residents have until November 21 to support the campaign by signing the petition on Lambeth Council's website.