Johnson said building two more runways west of London's busiest hub, a plan supported by the national Transport Select Committee, would cause "unacceptable degrees of noise pollution."
The Daily Telegraph reported Friday that Johnson also quipped that it would "probably be cheaper to move London slightly to the east," than to build the two new runways, which were targeted for a site 2 miles west of Heathrow.
The mayor's comments were made after British MPs rejected the mayor's proposal to build a new four-runway airport in the Thames Estuary that has been dubbed, "Boris Island," the BBC reported.
"They certainly have unfairly discounted the advantages of (Thames Estuary) plan, but what they are saying that's interesting is that the long-term future is not just for a third runway, but is for a fourth runway at Heathrow, indeed moving the whole shebang slightly to the west and constructing a really colossal four-runway hub airport," Johnson said.
"The difficulty is that that (the Heathrow plan) would really consign hundreds of thousands -- if not millions of Londoners -- to unacceptable degrees of noise pollution," he said.
The Thames Estuary proposal has been criticized as potentially harmful to migrating birds.
The MPs, led by Louise Ellman of the Labor Party, also said Johnson's plan would cost $41 billion, far more than the Heathrow expansion plans that are expected to cost about $18.4 billion.