A Department of Health spokesman said: "The NHS is now urgently investigating and if any wrongdoing whatsoever is found, including criminal actions, we will want to see the police and relevant NHS regulators alerted as necessary."

He described the claims as "clearly completely unacceptable" if found to be true.

Simon Douglass, medical director of the London Central and West Unscheduled Care Collaborative, which runs the centre, said: "I would like to reassure patients and the communities that patient safety is, and always will be, our highest priority.

"We take any allegations extremely seriously and have launched an urgent investigation. The matters leading to the allegations appear to have arisen earlier this year when the undercover reporter participated in training.

"All issues relating to staff conduct are extremely important to us, particularly in relation to patient safety. We strictly enforce our policies for managing staff conduct at all times including whenever we receive reports of inappropriate conduct or behaviour.

"We are working closely with the NHS clinical commissioning groups to ensure that any concerns are thoroughly investigated and we have appointed a senior investigating officer to oversee this process.

"The Sun did not share their evidence prior to publication and we will be asking them to disclose this to us to assist our investigation.