The prime minister is travelling to the G20 gathering of world leaders in Johannesburg in South Africa.
The summit brings together the 20 biggest economies, although Donald Trump has decided not to attend over widely discredited claims that white people are being persecuted in the country.
Sir Keir Starmer, whose critics label him "never here Keir" because of the frequency of his international trips, will emphasise the benefits of a prime minister acting as an ambassador for UK businesses abroad.
Sir Keir will visit a Johannesburg depot to see trains that were built in Derby and announce a new deal where the UK will "provide strategic advice and consultancy services" to South Africa's railways.
An organisation called Crossrail International, which is owned by the UK government, will carry out the work.
