Nick Clegg hopes to persuade his reluctant party on Tuesday to abandon its opposition to new airport capacity in the London area, a move that could allow the Liberal Democrats to back an extra runway at Gatwick.
The party has long opposed any airport expansion on environmental grounds and will restate its explicit opposition to a new runway at Heathrow, whose flight paths take in swathes of Lib Dem voters in southwest London.
But Mr Clegg does not want his party to go into the next election promising to block any new runways in the southeast, a position that could prove awkward to hold in a post-2015 coalition with either the Tories or Labour.
So the Lib Dem leader is backing a compromise amendment that notes improvements in aircraft fuel efficiency and the introduction of quieter engines, suggesting that might make it acceptable to build a runway.
Mr Clegg is anxious to sweep away any potential “unexploded bombs” in the Lib Dem manifesto that could detonate in a coalition, avoiding a repeat of the party’s damaging U-turn over university tuition fees in 2010.
The issue of airport capacity will come to a head shortly after the 2015 election when Sir Howard Davies’ airport commission recommends whether London’s capacity shortage should be addressed by a new runway at Heathrow or Gatwick.
The Lib Dems are not prepared to budge on their opposition to a new runway at Heathrow: the issue is effectively a “red line” in any coalition negotiation, although Mr Clegg declines to use that phrase.
But there will be strong business pressure on politicians to act on Sir Howard’s recommendation, ending decades of indecision on London airport expansion; Mr Clegg fears being portrayed as a block on a vital investment decision.
The amendment backed by Mr Clegg would scrap the wording in the party’s draft manifesto that excludes a new runway at Stansted or Gatwick, inserting qualifying phrases about the need to meet emissions targets and acceptable noise limits.
“It’s going to be tight but we think we can win it,” said one Lib Dem official.
Mr Clegg will not speak in the debate, but has let it be known that he supports the amendment.
The chief executive of the country’s second biggest airport operator re-entered the debate when he said that Britain does not need a hub airport and Heathrow should not be allowed to build another runway.
Charlie Cornish, of Manchester Airports Group, which owns Manchester and Stansted near London, has previously said he wants a second runway at Stansted by about 2030 but remained silent on the divisive question of Heathrow expansion.