The Department of Health has described as "shocking" newspaper claims that an unregistered doctor is providing medical assessments to patients who want to go to Dignitas.
The Daily Mail reported that the Swiss suicide clinic was referring patients who wanted to end their lives to a British psychiatrist who was struck off the medical register nine years ago for serious professional misconduct.
An undercover reporter posing as a young woman with mental illness was advised by staff at the clinic that Colin Brewer "should be her first try" if she wanted to end her life.
They told the reporter he was "open minded" about mercy killing and was prepared to support cases even where the patient was not terminally ill.
When she approached the psychiatrist, he said he would be prepared to assist her in her bid to kill herself, the newspaper reported.
Discussing her wish to die at 35, he reassured her that her age would be no obstacle since she was in her "middle life".
The newspaper said Dr Brewer had confirmed to it that Dignitas had referred around 12 patients to him for advice and assessments.
He pointed out that he was legally entitled to carry out such assessments and always made it clear to patients that he was not on the medical register - as he did in the case of the Mail's undercover reporter.
He added: "I do not provide treatment. Reports to Dignitas are carefully scrutinised by the Swiss authorities. Dignitas would not use mine if they were not accepted and respected by independent and rigorous scrutineers."
A Department of Health spokeswoman said: "This is a shocking investigation. We urge anyone experiencing depression to seek NHS care, rather than unregistered practitioners. In the UK, psychiatrists must be registered with the GMC in order to provide psychiatric assessments."