The Prince of Wales praised the NHS for withstanding the “pressure” of the pandemic as he opened a new facility providing ground-breaking cancer treatment.

Charles paid tribute to the “professionalism” and “resilience” of medical staff as he formally launched the site providing cutting-edge Proton Beam Therapy (PBT) and home to one of the largest treatment centres for blood disorders.

The University College Hospital Grafton Way Building in central London is a £380 million project that has been caring for patients since late last year.

After touring part of the building and meeting a young cancer patient midway through her PBT course, the prince gave an impromptu speech before unveiling a plaque to mark his visit.

He told staff, senior figures from University College London Hospitals NHS Foundation Trust and private supporters of the Grafton: “It is a remarkable achievement, and, for what it’s worth, I can only offer my congratulations to all those who played such an important part, let alone all those genius physicists who actually understand how the Proton Beam Therapy works, which is truly remarkable.”

He added: “I so well understand how much… pressure you’ve been under for the last two years or more, and quite how you’ve withstood it, I don’t know.

“It is a great tribute to your professionalism and your resilience that this has been possible, and for all of us who rely and depend on you and your skills we cannot thank you enough.

“And now you have to catch up with the backlog, which is the other great worry, but I’m sure you will do it.”

The PBT machine in the Grafton Way Building is housed in a huge basement which could accommodate the Royal Albert Hall and is one of only two sites in the country providing the treatment.

It also has eight operating theatres, with an accompanying 32-bed surgical ward and 36-bed recovery area.

During his visit, Charles met Karen Dawson, 43, and her 12-year-old daughter Louise, who is into day nine of a 30-day PBT course for a spinal tumour.

The prince and Dr Yen-Ching Chang, clinical lead for PBT, joined the pair, from Wickford, Essex, as they relaxed in a room where the schoolgirl was drawing pictures of rabbits ahead of her next session.

Charles was impressed by her efforts, asking: “You’re doing this just freehand? It’s very good – fantastic. Do you do a lot of drawing?”

Louise was a little tongue-tied and, when the prince asked about teaching, her mother said a tutor provides the core subjects of maths, English and science, whereupon Charles quipped to the young patient: “You will be an expert in proton beam.”

Charles appeared fascinated when Dr Chang, who is overseeing the schoolgirl’s PBT care, showed him the suite where the treatment takes place.

PBT is a type of radiotherapy that targets cancer tumours with proton beams, and the consultant oncologist told the prince “essentially it damages the DNA so that the (cancer) cell can’t divide in the future”.

Before leaving, Charles met staff who had lined the path to his car, and was presented with a multi-coloured face mask.