"Hard sell" tactics by the country's biggest provider of corrective laser eye surgery are leading to calls for far tougher regulation of the lucrative industry.

A leaked 10-page training manual from Optical Express, marked confidential, details how front-of-shop staff, known as patient advisers, are to "take control" of potential customers. Among its diktats, it says that staff should "look for a 'nugget' – something personal that you can share with them or that they can share with you.

"If you can find that personal connection, you can refer to it later and share it with your optometrist. A good nugget can be the difference between a booker and a thinker."

Optical Express is the market leader in laser eye surgery, but has been at the centre of a series of controversies. Earlier this year the high-street optician lost a battle to have a website that strongly criticised its record in laser eye surgery closed down over what it claimed were defamatory attacks funded by a commercial rival.

The company's lawyers tried to have the domain space of the site, Optical Express Ruined My Life, removed, but the bid was thrown out by Nominet, the internet registry company adjudicating on the complaint.

A recent investigation by the consumer watchdog Which? found that Optical Express failed on every one of its researchers' visits to explain possible complications with eye surgery, which include poor night vision, growths where the eye is cut, double vision and chronic eyelid inflammation.

Meanwhile, the Labour MP John McDonnell has been seeking greater regulation of the industry, in which the only legal requirement for someone to perform laser eye surgery is that they are a doctor registered with the General Medical Council. There is no requirement to hold a specialist qualification and there are no industry-wide statistics on injuries caused by the procedures.

The leaked training manual seen by the Observer tells Optical Express staff to take the "best care of your patient that you possibly can – it will help you book them and even if they do not book, they will recommend you, and Optical Express, to their friends and family – it is all about the experience!". Yet critics claim that the manual instead prioritises sales of treatments.

The firm, whose adverts promise perfect vision "from £395 per eye", suggests ways to identify those people that they call "Mr £395".

It says that staff should brief the optometrists who would carry out any operation about the "type" of patient they are handling "out of sight and hearing of patient", and include information on what "things to avoid" discussing with the customer for fear up putting them off, along with the aforementioned "nugget".

The document adds: "Your brief is the difference between a booker and a thinker – set your optometrist up to succeed!!

"If your patient has indicated that they wish to go away and think, it is essential that you establish the REAL reason why they are not going ahead – use the following structure to establish this:

"Confirm the reason: 'Mary was saying that you were a little unsure about the dates for treatment and wanted some time to think about it'.

"Empathise with their reason: 'I completely understand, you lead a busy life and need time to consult your diary – I am exactly the same!'"

But the firm suggests that if the prospective patient is a "thinker" they may need face-to-face reassurance. "Face your patient, put the pen down, and look them in the eye," the document says. "Use statements like: 'What price can you put on your eyesight?', 'Remember you do this for the rest of your life', ie Lifetime investment."

Sasha Rodoy, a campaigner seeking tighter regulation of laser eye surgery, said the leaked document confirmed all her fears about the industry's priorities. "These 'hard sell' techniques may not be illegal, but they are totally inappropriate for a medical procedure and they should be illegal."

A spokesman for Optical Express said that the manual was no longer in use by the company. She said: "Every patient who seeks laser eye surgery with Optical Express must first attend a highly detailed refractive consultation to determine if they are suitable candidates for treatment.

"Each member of staff involved in the consultation process undergoes extensive training and development to ensure we deliver an outstanding patient experience and clinical outcome. Our patient advisers help to put patients at ease and address any non-clinical concerns they may have while our qualified and highly experienced optometrists focus on clinical matters. More than 99% of our patients achieve 20/20 vision or better without spectacles or contact lenses post-surgery, based on a study of over 293,000 patients with the most common prescription ranges we treat."