Campaigners have warned against BAA introducing a controversial measure “by stealth” that would see planes landing and taking off from both runways at Heathrow.
Tim Hardy, airside director of BAA, said one runway could be used for mixed mode at certain times of the day when the Cranford agreement is scrapped.
The Government barred the introduction of mixed mode when it approved a third runway in January.
Mr Keen said he would cause "absolute hell over the dispute".
He added: “BAA are despicable if they are in any way thinking about it.
“I and Ann [Keen, MP for Brentford and Isleworth,] campaigned to stop any introduction of mixed mode and we were assured that it was going to be barred.
“I just do not believe it because the statement said no, it didn’t say possibly, it didn’t say only for a very short time, it just said no. I’m absolutely staggered.”
He said residents would suffer from double the amount of aircraft noise if mixed mode was fully introduced.
Critics of Heathrow expansion feared the abolition of the 57-year-old Cranford agreement - also announced by former Transport Secretary Geoff Hoon in January - would leave BAA open to bring in the measure.
John Stewart, chairman of campaign group Hacan, said: “What we will certainly be watching and making sure of is that this very short period of time doesn’t escalate into something approaching mixed mode.
“I think the danger is that this could turn into mixed mode by stealth.
“I think people won’t like it at all, but maybe prepared to accept it if it’s just for no more than 20 minutes or so during that day, but any more than that then there would be a real outcry.”
Mr Hardy also said at a Heathrow Airport Consultative Committee (HACC) in July that it would take at least 18 months before the abolition of the Cranford Agreement, which prevents planes taking off from the eastern end of the northern runway, would come into effect.
Hounslow Council has said it will lead to the town becoming one of Europe’s noisiest communities.
Mr Stewart said: “The Cranford agreement is and always has been the big block to mixed mode.
“The fact the Cranford agreement is going means it’s potentially open to bring in mixed mode by stealth.”
BAA said it believed the minutes of Mr Hardy's comments at the HACC meeting had not been taken accurately.
A spokesman for BAA said: "The Government said in January that there will be no mixed mode at Heathrow.
"When the airport is on westerly operations, there are times when the runway being used for departures is also used for a limited number of arrivals, and similarly the runway being used for arrivals can also be used for departures.
"This procedure is allowed within our operating restrictions and is only used under certain conditions.
"The ending of the Cranford agreement will enable us to fully mirror this when on easterly operations since we will be able to use the northern runway for departures as well as arrivals.
"This is not mixed mode, and the strict cap on the number of airline movements (480,000 per year) will not be affected."
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