They’re asking Heidi Alexander to reject proposals for a third runway there.
The letter follows recent reports that the Government plans to endorse expansion.
Chancellor Rachel Reeves said economic growth “trumps other things” in a speech at the World Economic Forum in Davos.
Reeves is expected to make an announcement backing airport expansion this week, as part of major speech on plans to boost the economy.
The letter was signed by Liberal Democrat MPs Munira Wilson, MP for Twickenham; Sarah Olney, MP for Richmond Park; Paul Kohler, MP for Wimbledon; Monica Harding, MP for Esher and Walton; and Joshua Reynolds, MP for Maidenhead.
Ahead of a formal announcement, Paul McGuinness, Chair of the No 3rd Runway Coalition, said:
“If, as is now expected, the Chancellor announces the go ahead for Gatwick’s and Luton’s expansions this week, then perhaps her supportive words about Heathrow are not as significant as some may think.
“The Airports Commission framed extra capacity as a choice between Gatwick or Heathrow, and not only is Gatwick’s expansion now likely to proceed, but so too Luton’s – joining Stansted and London City expansions to create extra annual capacity for 60 million more passengers. This is more than a third runway at Heathrow was ever going to offer!
“Expansions at other London airports undermine the case for Heathrow’s uniquely complicated and costly third runway, making it an even riskier, if not un-investable proposition.
“At least two of these expansions will be up and running before Heathrow’s even gets through the planning phase, and Heathrow hasn’t even submitted an application yet. Had these other expansions been in the pipeline a few years ago, the Airports Commission might not have recommended Heathrow’ s expansion at all.”