His son David Bennett Jr said: “He realises the magnitude of what was done and he really realises the importance of it.

“He could not live, or he could last a day, or he could last a couple of days. We’re in the unknown at this point.”

Mr Bennett had spent several months before the operation bedridden.

On Monday a heart-lung bypass machine, which had kept him alive before the surgery, was still attached to him.

But the new heart was working and the machine could be removed on Tuesday, doctors said.

Amid a shortage of human organs for transplants scientists have long been trying to develop techniques for how to use animal organs instead, a process known as xenotransplantation.

But previous attempts have failed after patients’ bodies rapidly rejected the animal organs.

In 1984 an infant patient in California known as “Baby Fae,” lived for 21 days after having a baboon heart transplanted.

Pigs are considered ideal donors due to their size, rapid growth and large litters, and because they are already raised as a human food source.

For the most recent attempt doctors used a pig heart from an animal in a herd that had undergone gene-editing.

That involved removing a particular sugar from cells that, in previous attempts, triggered a strong immune response and led to quick rejection of transplanted organs.

Dr Muhammad Mohiuddin, scientific director of the University of Maryland Medical Center’s animal-to-human transplant programme, said: “If this works, there will be an endless supply of these organs for patients who are suffering.”

Dr David Klassen, the hospital’s chief medical officer, said: “I think you can characterise it as a watershed event.”

Permission for the operation had to be given by the US Food and Drug Administration.

It was authorised on “compassionate” grounds because the patient had no other options.