The private healthcare company that performs controversial sickness benefit assessments has confirmed that it makes all its medical staff sign the Official Secrets Act.

Atos Healthcare admitted that doctors and nurses are asked to sign a document which sets out the basic terms of the Official Secrets Act even though much of the company's taxpayer-funded work is not bound by the terms of the Act.

Two doctors who work for Atos raised their concerns with the Guardian and online political blog Liberal Conspiracy after the company asked them to sign a document pointing out their obligations under the OSA.

Both doctors believed that signing the document would hinder people in the organisation from blowing the whistle on patient safety and issues around whether medical assessments were being carried out appropriately.

Though it would be highly unlikely for medical staff to be prosecuted under official secrets legislation for speaking out on patient safety issues, doctors are concerned that signing the document would itself make staff more reluctant to come forward.

Atos, a French-owned healthcare and IT company, is being paid more than £100 million a year to carry out the controversial work capability assessments for the government, which help determine whether claimants are eligible for sickness benefits.

The process has been dogged by controversy since it began to be piloted under the last government and complaints snowballed once the system began retesting all Incapacity Benefit claimants last April, to assess whether they should receive the new Employment and Support Allowance.

One doctor who joined Atos last summer said that all medical health professionals, MHPs as they are referred to within the organisation, are required to sign the document.

The doctor, who spoke to the Guardian on condition of anonymity, said. "I could not have been more surprised when I was told that everybody had to sign the OSA."

"There's something very sinister, cloak and dagger about it. It's weird that it would be used in the context of our work; it's not the same kettle of fish at all."

Signing a statement outlining their obligations under the act would make staff "reluctant to bring matters of concern to the attention of the wider public", the doctor said because of the potential "threat of sinister, treason-like charges".

"Mild mannered healthcare professionals would all be twitchy about speaking out."

The doctor added that there were already strict rules about disclosure of patient details so there was no need to invoke such draconian legislation. "You would have thought we would be covered by our professional codes of conduct. The GMC and the Nursing and Midwifery Council would take a very dim view of if we disclosed personal details."

A second Atos doctor also working on sickness assessments said he had concerns about the ability to speak out about systemic problems. "There is no justification for a healthcare practitioner assessing a person suffering from back pain or depression to sign this kind of document."

Atos has said that it has contracts with other public bodies including the Ministry of Defence which require its employees working with those bodies to sign the OSA.

Atos added that it has one blanket security policy across the company and this was why doctors working with the Department for Work and Pensions' assessment process were also being made to sign a statement saying they were aware of the terms of the Act.

"Atos Healthcare provides services to a number of clients and in line with our best practice security policy, we ask employees who handle confidential information as part of our work with public clients to sign a set of security compliance forms including the Official Secrets Act declaration form."

In a parliamentary answer to a question from Tom Greatrex, the Labour MP for Rutherglen and Hamilton West, in February the employment minister, Chris Grayling said his department did not require Atos employees to be bound by the terms of the OSA or extra provisions which bring further penalties for disclosing government information protected under the Act.

In response to a further parliamentary question, Grayling added: "No Atos Healthcare employees have been prosecuted for breach of confidentiality rules … from 2008 to date."

A Department for Work and Pensions spokesperson told the Guardian that the department was not aware that Atos employees were being made to sign the OSA.

Greatrex said: "If it is the case that Atos medical staff are required to sign the Official Secrets Act then this is an extraordinary development. This would prevent the possibility of problems with the system coming to light.

"It is only through transparency and accountability that real improvements to the work capability assessment can be made. When Atos is in receipt of millions of taxpayer money, it is unacceptable for a culture of secrecy and cover-up to prevail."

Concerned about continuing problems with the accuracy of the Atos-administered work capability assessment, Greatrex recently asked the National Audit Office to investigate whether the company provided taxpayers with good value for money. The NAO is conducting preliminary inquiries to establish whether to investigate.

Atos is understood to be tendering for new contracts to carry out more health tests for the government, and details will be announced next month.

A DWP spokesperson said: "We are satisfied that the Atos arrangements meet the confidentiality requirements specified in our service agreement."