The speed at which mandatory criminal checks are being completed by the Metropolitan Police is of "great concern", a government department said.
A Disclosure and Barring Service (DBS) check is needed to work with children or vulnerable adults.
The London force has 50,570 outstanding applications. The Met says that is down from 83,000 in April and more people have been brought in to process cases.
One nurse said she lost her job after an eight-month wait for clearance.
A DBS check is supposed to take up to eight weeks, with no checks taking longer than 60 days. But just over half of cases - 51.9% - are dealt with within 61 days and on average, it takes 107 days.
This is more than twice as long as the next worst performing police force, North Yorkshire.
The Met handles a significant proportion of DBS cases nationally but it has missed government targets on processing background checks for more than two years.
Among police forces across the UK, an average of the monthly average turnaround times for April to October 2016 is 11.8 days.
Its increased workload is said to be down to staff shortages and a surge in numbers of people applying for private hire licences who also need checks.
It means employers are having to cope with vacancies after having agreed to employ someone and individuals are losing income because they cannot start jobs until the checks have been made.
A spokesman for the DBS said: "We rely on the Metropolitan Police as an essential partner in processing the enhanced level element of DBS checks.
"The current delay is of great concern to us and our customers."
The effects are being felt widely in the capital among everyone from health workers, social care workers and teachers to taxi drivers, volunteers and foster carers.
Cathy Kerr, director of adult social services for Richmond and Wandsworth councils, said: "One of our local home care agencies in Richmond borough, a small company providing support to people in their own homes, currently has 37 new recruits waiting for their DBS clearance.
"That has a significant impact on their capacity to support people in their own homes."
The nurse who is now painting
Julie Stennett was offered a job heading a team of nurses immunising school children in south London.
While waiting for her DBS certificate to come through, she had to support herself and her teenage daughter using Job Seeker's Allowance and ended up having to use a food bank.
After holding her job for eight months, the NHS trust could not wait any longer.
"I'd been to university, I'd followed the rules and I'd got a job but I still wasn't able to go to Sainsbury's or Tesco's and put food in the trolley. You don't mind going hungry but you don't want your children to go hungry," she said.
She took a free course provided by the Job Centre to allow her to get painting and decorating work to pay the bills, having phoned the DBS repeatedly to explain she was in rent arrears.
"I'm so angry and so upset with the fact that these people just ignored everything I said to them," she said.
The Met has brought in extra people including those on restricted or recuperative duties. Transport for London workers have been seconded to deal with taxi driver applications.
In a statement the Met said: "Looking forward, we have put in place a robust resourcing plan that includes ongoing training, recruitment and productivity measures to ensure we build a resilient, sustainable team.
"The MPS recognises that applicants may be impacted by turnaround times and is working with DBS and the Home Office to make reductions in waiting times; however this needs to be balanced against making thorough checks and lawful disclosures."
Harriet Harman, Labour MP for Camberwell and Peckham, is among those who have been contacted by desperate constituents.
In March, the then Home Secretary Theresa May said in a letter to her: "I maintain a close interest in disclosure turnaround times and very much appreciate the impact that delays can have."
Mrs May said she hoped the letter conveyed "how much of a priority it is for me to resolve this issue as soon as possible".
This week a statement from the Home Office said: "Disclosure and Barring Service checks are an important aspect of public protection for both the voluntary and employment sectors.
"The DBS is working closely with the Metropolitan Police to ensure that their performance improves as quickly as possible."