GSK is selling 25 of its Australian brands to the South African company, including the herpes treatment Valtrex, the epilepsy drug Lamictal, the antibiotic Amoxil and antidepressant Aropax.

Sir Andrew Witty, GSK’s chief executive, said last month that he was looking for further ways to simplify the business.

The Australian brands being sold generated sales of around £83m in 2011 and £31m in the first half of 2012, GSK said. The sales are expected to complete in the fourth quarter of this year, subject to regulatory approval, and net cash proceeds are expected to be about £155m,

“Revenues for these products have gradually declined over recent years due to local market price reductions and generic competition,” the company said.

It added that the sales were part of its drive to sell low-growth or non-core businesses and to focus on “priority brands, products and pipeline opportunities that have long-term growth potential”.