A firm that will carry out capability assessments on benefit claimants for the government says it could take 18 months to reduce the existing backlog.
US-based Maximus Health Services will replace the current firm Atos in March 2015 on a three-year £500m contract.
There are currently more than 600,000 claimants waiting to be assessed; some have been on the list for months.
The president of Maximus, Leslie Wolfe, told the BBC: “Improvements won't happen overnight.”
She said: “It'll take some time to hire the health care professionals. The expectation is that in between 12 and 18 months, we should be able to catch up on the waiting times.”
Atos, which has been doing capability assessments for people claiming Employment and Support Allowance since 2008, is quitting its contract early after reaching an agreement with the Department for Work and Pensions.
Employment and support allowance is the biggest sickness benefit and is given to about two million people.
Claimants must undergo an assessment to decide whether they are eligible for the benefit and if so, how much they should get. Recipients receive at least £101.15 per week.
Ms Wolfe said the key to reducing waiting lists was hiring “hundreds” of health care professionals to carry out the assessments and to explain to claimants what the tests are for.
She said: “There is a need to better explain to people that they're not coming to an exam, they're coming to an interview.”
Initially, Maximus will use existing Atos staff, buildings and IT equipment.
Protests against Atos are reported to be part of the reason the company ended its contract, but Ms Wolfe said Maximus would not suffer the same fate.
She said: “We're hopeful that if we [conduct] the assessments in a timely manner, some of that anger and resentment will go away.”
However, in the past, Maximus has incurred fines for errors in the US and some doubts have been expressed over whether the company will be able to handle the new responsibility.
Ms Wolfe said: “A number of years ago we made some mistakes. Since 2006 we've had a new administration team, a new CEO. We haven't had any serious issues since that time.”
A spokesperson for Atos Healthcare said: “We are currently exceeding our contractual agreement with the DWP on the number of people we see and reducing the backlog month on month.
“The department are aware of the number of healthcare professionals we have delivering this service and how many people we can reasonably see each month.”