They were hauled before Hounslow Council 's licensing panel on Tuesday (August 2) for a licensing review which could have resulted in their closure.
Both stores kept their licences after agreeing to numerous conditions , which include placing magnetic locks at the entrance, having two staff on duty at all times and providing extra training for employees.
Police cadets visited 22 betting shops across the borough during an undercover operation led by Hounslow Council's licensing team.
A dozen bookmakers failed initial undercover test
A dozen were found to be breaking the law by allowing under-18s to gamble, and the Brentford Betfred store and Hounslow Jenningsbet branch also failed follow-up visits.
At both those betting shops, cadets were able to play fixed odds betting terminals unchallenged, and were only asked for ID when they approached the counter to request change.
A spokesman for Betfred said it had implemented a full review and strengthened procedures after failing the tests.
A Jenningsbet spokesman said it had also introduced voluntary measures ahead of the licensing hearing to prevent underage gamblers using the branch.
'Working to protect young from lure of gambling'
Hounslow Council's deputy leader Amrit Mann said: "Protecting our most vulnerable residents is key to creating a safer borough.
"A lot of work has been undertaken by our licensing team to ensure our youngest residents are not victim to the lure of these highly addictive fixed odds betting terminals, which are commonly known as the crack cocaine of gambling.
"Almost all the borough's bookmakers were tested to check if they challenged underage gamblers.
"This is first time either Jenningsbet or Betfred have been held to account by a council. The conditions placed on the licences of these two high street betting giants reflect our concerns about the effects that underage gambling can have on young people.
"While our work to ensure underage gambling doesn't happen in Hounslow will continue, we hope that Betfred, Jenningsbet and other bookmakers will pay more attention to who is attempting to bet in their stores as a result of these recent underage test purchases and the action we have taken in these cases."
Fixed odds betting terminals can be highly addictive, and concerns about their proliferation led councils to back calls for a £2 maximum stake.
A slot machine addict told getwestlondon earlier this year how he spent more than seven hours feeding machines at a Hounslow betting shop without once being approached by staff.