In January, airport owners Luton Borough Council revealed proposals to boost passenger numbers to 18 million per year.
But in March, operator London Luton Airport Operations Limited (LLAOL) said 15 million was more realistic.
The combined plans will now allow for a maximum of 18 million passengers, creating about 4,750 jobs.
London Luton Airport, the UK's fifth biggest, currently handled about 10 million passengers a year and is owned by the council under the name London Luton Airport Limited (LLAL).
Plans from both the owner and operator were put forward after claims the south-east of England would see a shortage of air travel capacity by 2020.
On revealing its original blueprint, LLAL said they needed a partner to help cover the cost and there was a possibility they might end the current deal with LLAOL in 2014, when it was due to expire in 2028.
The operator questioned the environmental impact of the expansion plan and said it would impose a termination payment on the council of several hundred millions of pounds if their contract was ended prematurely.
In a joint statement, LLAL and LLAOL said the combined proposal will "ensure the main elements from both proposals are retained" and the "increase in capacity will take place within the existing boundary and use the existing runway".
More details will be released when the plans are finalised in July and it is expected a planning application will be lodged by LLAOL later in the summer.
LLAL chairman Robin Harris said the two plans had "very significant similarities and synergies".
"By integrating these, we have listened to the feedback of residents and of partners and have arrived at what we feel is the most efficient and effective option to ensure the continued development of the airport whilst addressing environmental concerns," he said.
Managing director of LLAOL Glyn Jones said: "The airport has significant potential to deliver even more for the local community and wider region".
The new proposal would also enable the airport to continue to be operated by LLAOL until 2031.