Rail services from Waterloo Station will have a new operator for the first time in two decades when First Group and MTR take over the South Western franchise currently held by Stagecoach's South West Trains.
The seven-year franchise begins in August and the handover from the old operator to the new company will take place in the midst of a part-closure of Waterloo Station for major engineering work.
"First MTR South Western Trains Limited will deliver the improvements that people tell us they want right across the South Western franchise area, from Bristol and Exeter, to Southampton and Portsmouth, to Reading, Windsor and London," said transport secretary Chris Grayling.
"We are delivering the biggest rail modernisation programme for over a century and this franchise will deliver real changes for passengers, who can look forward to modern trains, faster journeys and a more reliable service."
Planned improvements under the new franchise include more frequent services on Sundays.
Rail unions have criticised the involvement of MTR – operator of the Hong Kong Metro and future operator of London's Elizabeth line – in the new franchise.
RMT general secretary Mick Cash said: "Once again the Government have refused to consider the public sector option for a major rail franchise and instead it's a foreign state operator, in this case the Chinese state, which is set to make a killing at the British taxpayers expense.
"The nonsense is that with the Government triggering Article 50 this week they would be free to ignore EU rail directives that slam a block on public ownership. It is frankly ludicrous that the Tories are continuing with the 'any state but the British state' policy which has plundered our railways for over two decades.
"RMT is deeply concerned at exactly what this announcement will mean for our members, these crucial rail services and the safety of the travelling public. We will be seeking an early meeting with the new owners to secure cast iron guarantees on the jobs and role of the guards, the future of the wider workforce and the safety and quality of passenger services."