The mother of a terminally ill boy smothered to death by his father in a "mercy killing" was banned from driving today for being more than twice the drink-drive limit.

Mary Richards, 43, was spotted by a member of the public driving her white Audi in a "hesitant manner" in Goring, on November 12 last year.

Richards had been under stress that day as she collected her nine-year-old son George from his grandparents, with whom she is involved in legal proceedings against over access to him, magistrates heard.

Andrew Walker, prosecuting, told Worthing Magistrates' Court that the member of the public followed her car before police arrived on the scene.

He said: "They took a roadside breath test which was positive. She was arrested and taken to the police station, and gave two further positive breath tests."

The lowest of the two tests revealed 85mg of alcohol in 100 millilitres of breath, Mr Walker said. The legal limit is 35mg.

Proceedings against Richards were previously adjourned for experts to explore whether medication she was taking affected her drink-drive reading.

But Philip Norton, defending, said scientific analysis had since failed to support any defence and Richards pleaded guilty to driving with excess alcohol today.

Richards is understood to live still at the home in Henty Close, Worthing, where her former husband Andrew Wragg suffocated their 10-year-old son Jacob in 2004.

Jacob suffered from the rare genetic disorder Hunter Syndrome, which leaves its victims deaf, incapable of speech, incontinent and eventually in a vegetative state before death.

Wragg, a former SAS soldier, was sentenced to a suspended two-year jail term in December 2005 after admitting manslaughter.

A jury at Lewes Crown Court cleared him of murder after hearing him claim that the pressure of looking after Jacob, his failing marriage and horrors he witnessed in Iraq led him to a state of mind in which he believed his son had "come to the end of the road".

Mr Norton handed magistrates Richards' unblemished driving licence featuring her previous surname of Wragg, which he said would be familiar to the bench.

Making reference to the Wragg case, Mr Norton said: "Since that time, she has been engaged in High Court proceedings in relation to the grandparents concerning access to George.

"On this particular day, George was having contact with the grandparents and Mrs Richards went to the grandparents to collect George, so it was a very stressful day.

"Whilst at the house, she had some red wine. The glass was topped up and then she left and was stopped by police. She has accepted that she was over the limit."

Mr Norton said Richards was not working, and was on long-term incapacity benefit.

Chairman of the bench Graham Toole-Mackson disqualified her from driving for 20 months, fined her £250 and ordered her to pay £43 costs.

She could have her ban cut by five months if she passes a Drivewise course by April 13 next year, Mr Toole-Mackson said.

Richards left court without comment.