The Healthcare Commission and the NHS' National Specialised Commissioning team are now conducting a review of the hospital's heart transplant work.
The investigation which will report at the end of October will review the care of patients who died between January and September this year. It will determine whether the care that the patients received was appropriate and whether there were any common factors that could have contributed to their deaths.
Professor Robert Bonser clinical director of heart and lung transplantation at University Hospitals Birmingham foundation trust and Dr Jayan Parameshwar consultant transplant cardiologist at Papworth hospital foundation trust have been drafted in to carry out the investigation.
The Healthcare Commission's head of investigations Nigel Ellis said: 'We must remember that heart transplantation is extremely high-risk surgery…nonetheless, there are a range of measures that can be put in place to mitigate that risk and we want to make sure these are in place at Harefield hospital'.