A fraudulent doctor who worked in the NHS for 20 years has been convicted of a “deliberate and wicked deception” after forging a medical degree certificate and practising as a psychiatrist without the necessary qualifications.

Zholia Alemi, who is believed to be 60, worked in hospitals across England, Wales and Scotland and received income and benefits of more than £1m across her career, Manchester crown court heard.

On Wednesday, Alemi was found guilty of 13 counts of fraud, three counts of obtaining a pecuniary advantage by deception, two counts of forgery and two counts of using a false instrument.

The judge Hilary Manley remanded Alemi into custody and told her she would face a prison term “of some substantial length” when she is sentenced at the same court on 28 February.

She said the deception carried out against health authorities enabled Alemi to work with “potentially very vulnerable people over a long period of time”. The judge questioned how Alemi had been able to practise for as long as she had and in so many positions.

Sending a forged certificate to the General Medical Council in 1995, Alemi claimed to have qualified at the University of Auckland in New Zealand. A forged letter of verification referred to “six years medical trainee with satisfactory grade”. Yet official records showed that she completed only the first stage of the degree and was stopped from re-enrolling after failing a number of years. The court was told that in the letter of verification, the word “verify” was spelled as “varify”.

Between 1998 and 2017, Alemi worked at various health bodies and trusts, at times was employed by the NHS, and secured positions through recruitment or staffing agencies.

Christopher Stables, prosecuting, told the jury that Alemi had worked with patients across “quite literally the length and breadth of the country”. He said: “A conservative estimate, and I stress a conservative estimate, as to the quantum – so the overall amount of money fraudulently obtained by the defendant from the NHS – is somewhere, the prosecution say, between £1m and £1.3m.”

Over the four-week trial, the jury was told that Alemi had been convicted of three fraud offences at Carlisle crown court in 2018 after forging an 84-year-old woman’s will to make herself the beneficiary and forging powers of attorney.

She was sentenced to five years in prison in that case and the GMC apologised for “inadequate” checks and “any risk arising to patients as a result”.

Stables described Alemi as “a most accomplished forger and fraudster” who had “no qualification that would allow her to be called, or in any way to be properly regarded as, a doctor.”

Janice Wild, of the Crown Prosecution Service, said confiscation proceedings would be pursued aiming to recover the criminal property from which Alemi has benefited.