Siobhain McDonagh has launched a petition calling for an end to the "Bobby Tax" on anyone who wants to join the police force.
The MP for Mitcham and Morden quizzed David Cameron during Prime Minister’s Question Time on Wednesday about why applicants have to pay £1,000 before they enrol.
She said: "A £1,000 bobby tax will make it harder for the police to look like the community that it serves and I represent. It will put off young people from poorer backgrounds and ethnic minorities from joining the police."
After rallying young people from Mitcham and Morden to attend a police recruitment drive in Wimbledon last September, Ms McDonagh saw them lose interest as soon as they heard about the fee.
She said: "Everyone agrees we need to have a police force that is more like the community it polices. We need more people from tough backgrounds, more people from ethnic minorities. In short, we need more people from places like Mitcham and Morden."
Close to 1,000 people have already signed the petition, which comes a week after the Mark Duggan verdict opened up the debate on representation in the police.
Ms McDonagh believes a more diverse police force and more police on the streets would improve public confidence in Mitcham and Morden police and encourage people to report crimes.
She said: "There used to be one neighbourhood safety team per ward, so there was a locally-based team that spent the whole time in the area.
"They were your local bobby on the beat and they would never move out of the borough. Now safer neighbourhood teams work all over the borough."
The £1,000 charge must be paid to attain a Certificate in Knowledge of Policing a pre-application training course.
David Cameron has said he will discuss the fee with the Home Secretary.