A new company given the contract to assess disabled people for a sickness benefit has told the BBC it will do one million assessments this year.
Maximus is being paid £595m over three years to carry out work capability assessments for people applying for employment and support allowance.
The Department of Work and Pensions cut short a contract with Atos last year after "significant quality failures".
Maximus is promising to clear a backlog of around 600,000 claims.
It is also planning to reduce the time people wait for their results from at least 120 days to the recommended 90 days.
President of health services, Leslie Wolfe, told BBC Radio 4's You and Yours programme: "Part of what Atos didn't have was a [big] enough team to keep up with the wait times.
"That's one of our first priorities. We need to clear about one million [work capability assessment] claims this year.
"We'll actually need hundreds of new healthcare professionals across the UK in order to clear the backlog that's there, which is about 500,000-600,000 people, and also to keep up with the ongoing new volume of claims that customers are putting in."
Maximus says it is currently recruiting "hundreds" of clerical and administrative staff.
It will also double the number of specialist mental health champions from 12 to 24, who will carry out face-to-face assessments and train others to do them.
Ms Wolfe said another area it planned to improve on was communicating with customers.
Maximus is going to recruit more health professionals, like physiotherapists, but will not take on any more doctors.
Instead they will continue to employ the same number of doctors and contract GPs, which means the percentage of doctors to health professionals will decline.
Atos was appointed by the Labour government in 2008 to carry out assessments, but was dogged by controversy as the number of people wrongly assessed as fit for work grew.
At the same time, increasing numbers won their appeals at tribunal.
Some people who had been told they were fit for work and should find a job later died.
Radical change urged
Maximus has employed disability rights activist Sue Marsh as its head of customer experience.
She blogs at Diary Of A Benefit Scrounger and is a member of the campaigning organisation We Are Spartacus.
Ms Wolfe said Ms Marsh would be "constantly looking at different things we can improve".
She also said that, as with Atos, the final decision on whether someone was fit for work, or should be put into the work-related activity group in order to be helped into work, or a support group, would be made at the relevant JobCentre Plus.
Disability Rights UK has been recruited to help with training and re-training.
Its board said in a statement that it believed the work capability assessment system needed radical overhaul and was "simply not fit for purpose".
"It is often experienced as punitive, it has no validity and the way it has been delivered is far too often inaccessible, disrespectful and distressing," it added.
Mental health charity Mind said it would like to see Maximus have more assessors with mental health expertise.
And the Multiple Sclerosis Society said it was "vital" that Maximus improved the record of the work capability assessment for people with "fluctuating conditions".