The National Union of Teachers is the latest organisation to show concern about the possibility of storing liquefied natural gas on Canvey.
A representative from the organisation is believed to have attended a public meeting held by protest group People Against Methane (Pam) in Richmond Hall, Benfleet.
The meeting was set up in an effort to form a committee from the Benfleet area to join the chorus of protesting voices already gathering pace on Canvey.
It is thought the union is concerned with the safety of pupils in schools and the lack of emergency plans that have been made available for teachers in the event of a disaster striking the gas tanks, which would hold 108,000 tonnes of the liquid gas under the proposals.
Castle Point Council refused the plans for the expansion in September, but the Calor Gas consortium behind the application have until the end of March to appeal.
Jerry Glazier, the union's executive member for Essex, confirmed the issue was "something that the local association is aware of" but little had been discussed on the matter.
Mr Glazier said he had not personally authorised anyone from the union to attend the meeting, and declined to comment further.
George Whatley, chairman of Pam, said: "The NUT have shown concern over the safety over the safety of their schools in the light of top-tier installations within their locations and the fact that there are no off-site emergency plans for the schools.
"Teachers need to know what to do in the event of an emergency."
About 20 residents attended the meeting on Tuesday night and most have signed up to help Pam with tasks such as leaflet dropping and other publicity.
Mr Whatley added: "We have achieved what we set out to do at the meeting, which was to start up a delivery service and form a committee in Benfleet.
"This is still growing, and Benfleet people know Benfleet better than Canvey people. Our aim is to cover the whole of Leigh, Southend, Rochford and Rayleigh."
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