A mass protest against a third runway at Heathrow could cause hundreds of flights to be cancelled next week as demonstrators attempt to blockade Terminal 1.

BAA, the airport operator, has been trying to identify the leaders of the protest and has resorted to negotiating with environmental activists through the Facebook website.

Geoff Hoon, the Transport Secretary, is about to approve the £13 billion third runway next week after overcoming objections from members of the Cabinet, The Times understands. Although the decision to approve the project, which would allow BAA to submit a planning application next year, will provoke a substantial Labour rebellion, the risk of a Cabinet resignation has receded after sources said that Hilary Benn, the Environment Secretary, would not quit.

Climate Rush, the organisation that broke through police cordons and banged on the doors of Parliament in October, is planning to block the departure gate at Terminal 1 on Monday evening. It intends to occupy the terminal, from which 13,000 passengers depart each day, until the Government announces its decision.

Climate Rush expects at least 1,000 people to attend the protest and has advised them to pose as passengers. The protest is being advertised on the Climate Rush website as a “peaceful picnic” starting at 7pm.

The website states: “Our argument is not with those who must fly. To this end we shall do our utmost to allow airport users to go about their business.” However, The Times understands that this is a disingenuous statement made to avoid the risk of BAA obtaining an injunction against the protest.

The organisers, some of whom were involved in planning the runway invasion at Stansted airport that caused 50 flights to be cancelled last month, intend to block the departure gate and terminal entrances. One said: “If BAA has heavy security checks at the entrances, that will delay thousands of passengers anyway and the disruption will attract the media attention we seek. It is being presented as a peaceful picnic but people are prepared to go much farther when they are in a like-minded crowd.”

A second protest is planned at the same time at Manchester airport but is likely to be much smaller.

The 700 homes in Sipson, the village near Heathrow that would be demolished to make way for the runway and terminal, have been sent leaflets about the protest. Some of the 2,000 residents have attended training days on “direct action” tactics and are expected to attend the protest.

Climate Rush has chosen to protest at Terminal 1 because a large proportion of its passengers take domestic flights, which the protesters say could be replaced by less environmentally damaging trains.

This week, BAA sent messages via Facebook to Climate Rush organisers, including Tamsin Omond, 24, a Cambridge graduate who was fined in November for occupying the roof of the Palace of Westminster in February. Miss Omond, the granddaughter of a baronet, also took part in the attempt to rush Parliament in October. Many protesters were in Edwardian dress and said that it was a 100th anniversary re-enactment of the Suffragette Rush organised by Emmeline Pankhurst. Some of the Heathrow protesters on Monday plan to wear Edwardian clothing beneath their coats, revealing their costumes when a string quartet begins playing.

Damon Hunt, BAA's head of media, wrote on Facebook on Monday that BAA had already tried other means of contacting the organisers. “Unfortunately, no response has so far been forthcoming. I would therefore urge you to contact myself to ensure that the protest will be a well-organised, peaceful one, and that any unnecessary disruption of our passengers is avoided.”

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