Perverting the course of justice carries a maximum sentence of life imprisonment, although the average sentence is around 10 months.
Both 58-year-old Chris Huhne and economist Vicky Pryce, 60, have been warned to be under no illusion about what punishment they face.
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Armed police were out in force as runners competed in the largest public event in the city since Monday's terror attack.
Taliesin Myrddin Namkai Meche, 23, was stabbed along with Rick Best, 53, when they tried to stop a racist attack on two Muslim girls.
The shadow home secretary, 63, said she worked in the Home Office for nearly three years when she was a trainee.