Airport capacity at some south-east England airports, including Luton and Stansted, is underused, according to a London Assembly report.
Research commissioned by the assembly showed that in summer 2012: around 47% of Stansted airport's available runway take-off and landing slots were not used; around 5% of Luton airport's available slots were not used; 12% of slots at Gatwick airport were available.
To encourage passengers to switch from Heathrow, the report said improving transport access from central London to Gatwick, Luton and Stansted was needed - for example, by better rail connections and actively promoting public transport.
The report said bosses at Stansted reckoned they could attract 1.5 million more passengers per year if the rail journey time from London was reduced from 45 to 30 minutes.
The report also revealed that in 2010, as many as 127 million people used London's airports and most, including those using Heathrow, flew direct to their destinations rather than use the airports to transfer.