Teachers committed to giving their pupils learning opportunities outside the classroom and taking them on school trips are being "pressured" to do it during holidays and weekends in order to work around the "rarely cover" rule.
The details follow the publication last week of a Commons schools select committee report, which said children are at risk of becoming "entombed in their homes".
Its release coincided with the Association of Teachers and Lecturers' conference, where a primary teacher said she was forced to take her pupils on their outing at the end of the school day because of "rarely cover".
Carly Prout, an English teacher at a Berkshire primary, told the conference she now has to take school trips at 4.30pm. She said she recently took her pupils to a printing press but could not get home until late in the evening.
Schools have been abandoning educational trips to museums and galleries because heads are being overly cautious that teachers are not providing classroom cover on a regular basis.
Changes to the 2003 workload agreement resulted in a raft of contractual amendments to ensure that teachers were making the best use of their non-contact time rather than covering for absent colleagues.
The select committee report said there was evidence of school trips being cancelled as a direct result of rarely cover, even when bookings had been made "well in advance" and cover could have been arranged.
Speaking to the committee last month, Robert Lucas, chief executive of educational charity the Field Studies Council, said there had been a "significant reduction in bookings" at its 17 centres.
He said teachers were being "pressured to go during holidays and weekends" in order to work around "rarely cover".
Committee chair Barry Sheerman said: "The steep decline in the amount of time children are spending outside is shocking."
"Research has shown that the likelihood of a child visiting any green space has halved in a generation."
Rarely cover is a national agreement that was brought in through progressive contractual changes to the 2003 Workload Agreement. It included reductions in teachers' overall hours, new arrangements for deploying support staff and a limit on the amount of cover a teacher would be expected to employ in the instance of an absent colleague. The changes came into effect in September last year.