He is the first English cricketer ever to be convicted of on-field corruption.
Judge Anthony Morris adjourned sentencing until February 10 and warned Westfield: “I hold out no promises to you as to the eventual outcome of this case.
“It’s open to the court in this case to pass an immediate custodial sentence.”
A police spokesman told the BBC the prosecution was the first use of prevention of corruption legislation relating to a sporting event.
Det Sgt Paul Lopez said: “This was a ground-breaking case and highly complex.
“We are pleased that Mervyn Westfield has admitted the charge and we hope that this sends a strong message to professional sportsmen and women around the country.”
“If they intend to get involved in spot fixing, or think that match fixing is not a crime, then they need to think again.”
Former Pakistan captain Salman Butt, 27, was jailed for two-and-a-half years for his part in a spot-fixing betting scam that has engulfed cricket last year.
Pakistan teammates Mohammad Amir, 19, and Mohammad Asif, 28, were also given custodial sentences, of six months and a year, respectively.
All denied involvement.