His comments came after the government launched a commission on how to increase the UK’s aviation capacity, amid fears business is losing out.
Mr Cable told BBC One’s Andrew Marr Show the value of this exercise was to “look at the alternatives”.
Several senior Tories say Heathrow must expand, but others, including London Mayor Boris Johnson, oppose the idea.
Environmentalists and many residents of west and south-west London have raised fears over pollution, noise and damage to the area’s way of life.
A commission chaired by ex-Financial Services Authority boss Sir Howard Davies to examine ways to expand airport capacity will report in 2015, leaving the decision to the next government.
‘Political commitment’
The 2010 coalition agreement between the Conservatives and Liberal Democrats included not expanding Heathrow. The previous Labour government backed a third runway, but now opposes the idea.
However, there have been growing calls among senior Tories for a change of policy, with supporters arguing that UK business is losing out to international rival “hub” airports, such as Schiphol in the Netherlands.
But Mr Johnson, who opposes the expansion of Heathrow, has called the decision to set up a commission a “fudge”.
And Tory and environmental campaigner MP Zac Goldsmith, who represents Richmond Park and North Kingston, in south-west London, has threatened to resign if the Conservative Party changes its policy.
Mr Johnson is understood to have discussed the idea of standing in any resulting by-election as part of a bid to fight plans to expand Heathrow Airport.
Liberal Democrat Mr Cable, who also represents a south-west London seat, Twickenham, said: “This is not a parochial little problem for south-west London. There are potentially two million people affected by this.
“There’s an absolute political commitment not to expand Heathrow.”
He added: “It’s not going to happen, so the value of the commission that the prime minister has looked at is looking at the alternatives.”
‘Difficult debate’
These could include Mr Johnson’s proposal to build a new airport east of London, partly on reclaimed land in the Thames Estuary.
The new transport secretary, Patrick McLoughlin, has said the commission – headed by former Financial Services Authority boss Sir Howard Davies – will identify and recommend to government “options for maintaining this country’s status as an international hub for aviation”.
In a written statement, Mr McLoughlin said: “This is a very difficult debate, but the reality is that since the 1960s Britain has failed to keep pace with our international competitors in addressing long-term aviation capacity and connectivity needs.”
The Davies commission will publish an interim report by the end of 2013, with ideas on how to improve the use of existing runway capacity over the next five years and an assessment of what is needed to maintain the UK’s global hub status.
That will be followed, in the summer of 2015, by its final report.
