In an exchange with Philippa Jackson, representing Stella English at employment tribunal, Lord Sugar accused his former employee of lying about her version of events to generate publicity.
When Ms Jackson suggested he appeared angry, he said: "I'm angry because of this, because it's a total lie".
"When I came into the process of this tribunal, I was going to try to be generous to your client, but I've sat here and listened to her over the past two days and there is no other way to describe what she has said than as blunt lies."
Lord Sugar has accused one of his Apprentice winners of lying and taking him to a tribunal "to extract money". Stella English won the TV show in 2010, but is suing the businessman for constructive dismissal.
ITV News UK Reporter Sejal Karia is at the East London Employment Tribunal.
During cross-examination today 2010 Apprentice winner Stella English was asked by Seamus Sweeney, representing Lord Sugar, why she did not complain to the millionaire business mogul about how she was treated.
She said: "I was in a trial period to win The Apprentice.
"What value is there in me going into a company I don't know - whether I knew (she was going to win) or not - and then go and complain about people who have been working there a long time?"
The tribunal also heard that Ms English later asked Lord Sugar about how to get media advice on doing some public speaking or writing a book as she needed to make some money as she was in debt.
Ms English, who took home £82,500 a year in her previous job working for a Japanese investment bank, said Lord Sugar told her it would be "better all round that you didn't walk out".