Labour has taken Britain's largest train operator into public ownership.
The nationalisation of Govia Thameslink Railway (GTR) was heralded by Transport Secretary Heidi Alexander as a "defining moment in our reform of the railway".
GTR services carry more passenger than any other operators in the UK and account for one in six train journey in Britain.
Lines which will now be publicly owned include Southern, Thameslink, Great Northern, and Gatwick Express, covering large areas of south east England and beyond.
The Department for Transport (DfT) has already committed to doubling the frequency of Gatwick Express services between London Victoria and Gatwick Airport from December.
It will also introduce more early morning services on Saturdays and Mondays from this summer.
The government has also upgrade signalling between Farringdon and Blackfriars, to crackdown on graffiti in Thameslink toilets, and to deploy 110 new travel safe officers in an effort to tackle anti-social behaviour across the network.
Mrs Alexander said: "From this Sunday, millions of passengers across the South East and East of England will be travelling on rail services back in public hands - run for the public good, not private profit.
"Bringing Britain's largest train operator into public ownership is a defining moment in our reform of the railway.
"It gives us an opportunity to tackle the bread and butter issues people want, like driving down cancellations and improving the frequency of services to Gatwick Airport.
"As we set up Great British Railways (GBR), we're putting passengers first, fixing what's broken, and delivering a railway people can rely on."
GBR will be a new public sector body which will bring together responsibility for tracks and trains under a single organisation for the first time since privatisation in the mid 1990s.
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The first train displayed GBR branding was a Southern train unveiled in Brighton earlier this month after legislation to create the body was included in the recent King's Speech.
The move follows an announcement from the Government to upgrade WiFi services on trains across the country.
Satellite connectivity will be rolled out to all nationalised main line services through funding worth £57million following a trial held across LNER, South Western Railway and Great Western Railway.
GTR chief operating officer John Whitehurst said: "This is a railway that carries millions of people to work, to school, and to see friends and family every single day.
"From this Sunday every one of them will be on a publicly owned service, which is a responsibility we take seriously and one we have been preparing for.
"We have spent the past year building the foundations, and bringing even deeper integration into our operations with Network Rail, with a single focus on what’s right for our customers and communities."
GTR has become the fifth operator to enter public ownership under the Government, following c2c, Greater Anglia, South Western Railway, and West Midlands Trains.
Travellers on the line are unlikely to feel major changes immediately, but there will be temporary cuts to two per cent of services between July 18 and August 29. Fares will not rise after the Government implemented a freeze until March next year.
Chiltern Railways' services will be the next to enter public ownership on September 20, followed by Great Western Railways on December 13.
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