Kevin Wilson, 20, was due to appear in court on Thursday charged with GBH and baby destruction as Miss Bantala remained "critical" in hospital.
The university graduate was attacked outside her home in Peckham, south east London, by two men who kicked and stamped on her heavily pregnant stomach.
A neighbour dialled 999 at 8.16pm on Monday but more than an hour later at 9.25pm police were forced to cancel the request and take Miss Bantala to hospital in the back of their car.
A 40-year-old neighbour, who helped Miss Bantala after she was attacked, said: "The ambulance hadn't arrived after more than an hour.
"The neighbour who found her said, 'Look, she is pregnant' and the police ended up taking her.
"I helped her up and she thought she was bleeding so I helped her to my toilet.
"If the ambulance came earlier maybe they could have saved the baby."
Another neighbour added: "She said she felt wet and said I think my waters have broken."
The London Ambulance Service apologised late on Wednesday evening and confirmed the cause of the delay would be investigated.
They said: "We were called at 8.16pm on 15 June to Talfourd Place, SE15.
"We are very sorry that we were unable to send an ambulance before we were cancelled by the police at 9.25pm and are now looking into the circumstances of what happened."
Miss Bantala's mother Ida, a 49-year-old woman originally from Congo, said: "She was excited. She bought the baby's clothes and everything.
"She lost her baby. She was in pain and she was crying.
"She's in hospital doing okay but lost the baby this morning is so upset. She was happy. She wanted the baby."
Detective Chief Inspector Robert Pack said: "It is quite clear she was heavily, heavily pregnant.
"She was given first aid in the street and taken to hospital.
"That child was totally viable. The law says at 28 weeks the child is viable of independent life.
"It has had a purely devastating impact of that young lady's life and the sheer nastiness of the crime.
"The hospital did all the necessary checks, during the course of that night and the early morning no foetal heart beat was detected.
"She is fighting for her life, she is very poorly unfortunately and been transferred to intensive care overnight.
"She has friends and family around her and we are hoping she makes a full recovery.
"She certainly knows she has lost her child.
"She has been conscious and we have a specialist officer liaising with her in providing an account and a timeline."
