Thugs on a moped threatened a 10-month old baby with a “huge” knife in a terrifying street robbery on a young family.

Caroline Jimenez, 31, today told how one of the thieves held the blade to her baby daughter’s face as they stole her parents’ watches.

Ms Jimenez was with her husband, pushing their daughter in a pram, when they were targeted by two men in motorcycle helmets and balaclavas. She said: “We were just walking around Chelsea enjoying the sunshine, we’d been shopping and we entered the street because it looked nice.

“I saw two men, I thought they looked weird and I was a little scared but there were other people and I thought, ‘This is Chelsea in the middle of the afternoon. Nothing is going to happen.’”

The men followed them on the bike down Sprimont Place, just yards from Sloane Avenue near King’s Road, at about 3.30pm on Saturday. Ms Jimenez, a photographer, said “They passed next to us and I thought it was OK but then they came back and the passenger on the back started to run towards us with a huge knife. We started to scream, my husband stayed in front of him, and I started to run away with my daughter because I was pushing the pram.”

Her husband, 32, threw his watch at the robbers in a bid to get them to leave. “When I turned around the guy was coming towards me and then he put the knife to my daughter and I gave him my watch as well,” she said.

“[Our daughter] did not know what the knife was but she could feel we were upset and she cried. We’ll never forget the image of that man with the knife.”

The number of attacks by moped muggers has doubled in London in the past 12 months with a surge in thefts and assaults. Police are struggling to tackle the menace amid safety guidelines that make it difficult for officers to pursue mopeds through the crowded streets. Ms Jimenez, a Belgian national who came to London with her husband seven months ago, posted details of the attack on Facebook and called on Mayor Sadiq Khan to take action.

She said police arrived quickly but told her they were powerless to chase the criminals. “We have just moved here so I do not know what the rules are but I am furious the police cannot protect us,” she told the Standard.

“The police said we were the fourth people to be attacked that day. I just do not understand how the police cannot do anything, it is ridiculous. Now the anger is taking over. They knew exactly what they were doing — threatening a baby, the parents are going to do everything to protect her.”

A Met spokesman said: “Police are investigating a moped-enabled robbery.” The suspects, who fled towards Elystan Place, were white and wore black crash helmets, navy tracksuits and black Nike trainers. There have been no arrests.

Witnesses should call police on 101 or Crimestoppers anonymously on 0800 555111.