A company working to access NHS prescription data on behalf of pharmaceutical firms attempted to sign a gentlemen's agreement last year for access to the new central database of medical information on every patient in England, the Guardian has discovered.

The firm, i4Health, lobbied the new Health and Social Care Information Centre (HSCIC) in July 2013 for a memorandum of understanding to "ensure that requests [for patient data] from life sciences receive prompt attention", an examination of the stakeholder forums of HSCIC shows.

Senior NHS officials are due to appear at the parliamentary health select committee on Tuesday after plans to extract patient data from GP files were put on hold last week.

One of the key questions is likely to be how patient privacy will be protected given that, under the scheme, data from GP records – including children's mental health diagnoses, family histories and drugs prescribed, as well as smoking and drinking habits – would be sold to public and private research organisations, commercial companies, universities and "information intermediaries".

On Monday it emerged that an insurance society, Staple Inn Actuarial Society, said it was able to access the hospital records of 47 million patients over 13 years to help it decide premiums for customers.

According to the Competition Commission, i4Health will "offer access to NHS prescription data". One of the aims of the new firm is to allow drug companies to get together to access medical information easier than if they applied individually. i4Health has already made high-profile appointments, hiring the HSCIC's former chair Candy Morris as a non-executive director.

The pharmaceutical industry has championed the proposed "care.data" scheme, claiming that medical surveillance is key to making sure firms are getting the right drug to the right patients. Critics say the database's real worth is as an intrusive marketing tool.

By extracting patient data, drug makers will be able to know which drugs a doctor is prescribing and how that compares with others locally. They will also be able to discern whether patients are picking up their prescriptions and, crucially, will be able to gather details of patients' medical conditions and lab tests.

Companies argue that the current database used for investigating patient diseases, known as the Clinical Practice Research Datalink, is incomplete and the available pharmacy prescription data does not reveal patient conditions as GP records would – only the drugs they take.

Experts warned that the public would be uncomfortable with the idea of patient records being used to sell drugs. Ian Herbert, of the British Computer Society, and a member of the committee that advises the HSCIC on customer requests, told the Guardian: "Patient surveys show that the public get most upset when their records are used by commercial outfits for selling or marketing products. Many clinicians will not be happy about this too."

Under the care.data scheme, sensitive medical records containing NHS numbers, date of birth, postcode, ethnicity and gender will be extracted from every GP surgery in England, unless patients opt out. Patient data will be scrubbed of some of the most obvious personal identifiers – a process known as pseudonymisation – but not enough to make the information completely anonymous before it is sold to third parties.

"You have to keep the data pretty lean if you are going to keep the risk of identification at an insignificant level," Herbert said. "Once you start accessing it over time and comparing it with other datasets it quickly becomes quite a rich source of information. Then you can identify patients. That's the risk with big industries like pharma which have a lot of data."

Drugs firms deny there is a risk of patients' personal medical files being rifled through. Lawrence Berry, i4Health's chief executive, said: "Companies such as ours use anonymised data, not identifiable confidential data, to look for where improvements can be made to improve treatments and outcomes for patients."

Berry previously founded a medical information company, Datapharm, which is funded by 200 pharmaceutical companies. He said i4Health would be a non-profit firm whose members would include drug companies.

The HSCIC confirmed that i4Health had been looking to sign a memorandum of understanding with it last year to reduce delays in getting hold of patient data, but said the company would not now be asking for special treatment and would make requests like any other applicant.

However, campaigners claim that i4Health looks like a front for the drugs industry. Phil Booth, of the patient privacy campaign group medConfidential, said: "First NHS England officials flat out deny they are selling our data, even though money changes hands. Then they deny they sell data to insurers, but they've just been caught doing that.

"Now we find pharmaceutical companies are queuing up behind so-called not-for-profit front companies to spy on what pills we take to get better. The whole care.data scheme is starting to look like nothing more than a giant medical data-laundering machine."