The Metropolitan Police is planning to cut 3,500 civilian roles and 1,200 posts of the rank of sergeant and above.
The force is proposing the move to help save much-needed funds and comes on the back of news last month of plans to sell force HQ New Scotland Yard.
Despite the cuts, plans, which are subject to approval from the Mayor's Office for Policing and Crime, include a move to recruit 2,000 new police constables.
Jenny Jones, deputy chair of London Assembly's police and crime committee, said: "I am concerned the plan to cut a quarter of the support staff in the Met is a panic measure to stay within the budget and will result in more police officers in backroom roles.
"Over one in four support staff are going to be sacked, and at what cost in redundancy payments, in order to achieve the mayor's fabled target of wanting 32,000 police officers?
"And just how many of these police officers will be in backroom roles, not on the frontline, because of cuts in police staff?
"The Met already has 6,500 fully-fit officers in support roles and cutting the support staff so drastically, I worry, will increase this number.
"You cannot have the frontline without the back office."
Labour's assembly police and crime spokeswoman, Joanne McCartney, said the proposals will "jeopardise the safety and security of Londoners" and criticised Mayor Boris Johnson.
"This is further evidence that the government's cuts are hitting frontline policing in London," she said.
"Losing a quarter of our police sergeants will be a body blow to neighbourhood policing. They are our local police leaders who are absolutely critical in keeping communities safe.
"Boris cut 150 police sergeants last year, so the numbers are already too low.
"We need the right mixture of police officers, sergeants, and senior police leaders. These proposed cuts will fundamentally undermine this mix.
"Losing 3,500 civilian staff will also have a real impact on policing in the capital.
"We already know that police officers are filling back-office roles.
"These new cuts will exacerbate the problem and lead to fewer police officers on the beat. In the last two years the Met has already lost 1,956 police officers, 1,800 PCSOs and over 1,330 civilian staff.
"The Met are announcing these latest cuts without any consultation with Londoners and we can only assume that Boris fully supports them.
"The mayor and the government are cutting too far, too fast.
"They are jeopardising the safety and security of Londoners.
"It is time they listened to the public and abandoned these reckless plans."
A spokesman for the Mayor's Office for Policing And Crime (MOPAC) said: "For the first time in recent history we have a balanced budget so we know where the savings need to come from.
"This budget involved some difficult decisions on staff reductions, but these civilian staff will be offered generous voluntary exit terms and the most important result is that the Met will continue to have around 32,000 cops keeping London safe."