Sir Christopher Meyer, 74, was left covered in blood after the attack as he took the Tube home on Wednesday afternoon.
He was admitted to St Mary's Hospital in Paddington with injuries to his nose, left eye, and lip. An image of him in a hospital bed showed his head covered in blood and his eye swollen.
Sir Christopher's wife, Baroness Meyer, told the Times: "He looks terrible. His left eye is like a golf ball and bleeding, the nose looks like it could be broken.
"I’m absolutely shocked by the level of the brutality. They really beat him. It’s appalling — like something you would see in a war zone."
Police have arrested a man and a woman over the attack, according to Lady Meyer, who said she planned to raise the issue of violent crime in the House of Lords.
She said: "I’m absolutely shocked by the level of the brutality. They really beat him. It’s appalling — like something you would see in a war zone.
"He hasn’t had an x-ray yet. He doesn’t remember anything. The first thing he remembers is the police."
It came just a day before US President Donald Trump is due in London for a controversial visit which is expected to be greeted by mass protests.
Sir Christopher is an outspoken presence on social media, and has called for Britain to engage with Mr Trump rather than turn its back on him.
He served as British ambassador to the US from 1997 to 2003, coinciding with events including 9/11 and the preparation for the war in Iraq.
Lady Meyer added: "At first I thought that the attack was politically motivated. He is opinionated, and sometimes people have different opinions, but the police told me they believe that it is more likely that they might have wanted to rob him."
The British Transport Police have been contacted for comment.