An official watchdog is investigating five grouse-shooting estates for allegedly damaging the environment in a practice that they had pledged to stop.

Natural England is looking into allegations that the estates have repeatedly burned heather on their land to maximise the number of grouse for shooting. The watchdog launched its investigation after being passed evidence in the form of eyewitness accounts that the environmental group Friends of the Earth had collected.

The owners of the estates had voluntarily committed to ending the practice last year due to the threat of a compulsory ban. They categorically rejected the allegations laid against them by FoE, claiming that none was in breach of its voluntary commitments.

In a statement, Natural England, which is responsible for promoting nature conservation and protecting biodiversity, said it had been “investigating a number of reported incidents of burning taking place in upland areas. In a number of situations, we found that no further action was required. In others, we will continue further investigations and discussions concerning the management of these sites.”

Environmentalists have been pressing for a ban on the practice of burning heather, or rotational burning of blanket bog, as it is known. Blanket bog is a wet peatland habitat that is globally rare and threatened. The old heather is burned to expose new shoots – a food source that attracts grouse. Estates charge people who want to shoot grouse.

The watchdog’s investigation comes after Michael Gove, the environment secretary, was accused of letting the owners of large grouse moors off the hook over the practice.

The owners face the threat of a compulsory ban after the European commission launched an investigation into whether the UK government was failing to protect blanket bog habitats.

According to Whitehall documents, in February last year, Gove had suggested to the owners that they should stop the practice voluntarily to head off the possibility of a ban. The five estates were among more than 150 that signed up last year to the idea of voluntarily committing themselves to ending the practice. However, the evidence collected by FoE was gathered in October after the estates had signed up to the voluntary commitments.

The Moorland Association, a lobbying body that represents grouse moor owners, issued on a statement on behalf of the five estates. Its director, Amanda Anderson, said that where heather burning has taken place over blanket bog, it was to remove overgrown vegetation to enable the blanket bog to recover in accordance with government-endorsed guidance.

She added: “It is important to note that visitors to the uplands will continue to see smoke in the burning season as a result and that carefully implemented and legal burning on shallow peat will continue as a conservation tool. Burning is also used to reduce wildfire risk.”

The five estates are the Grimwith estate in the Yorkshire Dales, Midhope Moor, Hurst and Chunal Moors, and Moscar Estate in the Peak District, and West Arkengarthdale in the north Pennines, according to correspondence between FoE and Natural England.

A sixth estate, Walshaw Moor in Yorkshire, is also under investigation by Natural England, even though it did not sign up to the voluntary commitment, and instead reached an agreement with the watchdog to manage the estate’s environment. The estate also rejects the claim levelled by FoE.

The European commission launched its investigation in 2016 after receiving allegations that the Walshaw estate was burning heather on blanket bog.

FoE campaigner Guy Shrubsole said: “Burning moorland on rare blanket bogs wrecks ecosystems, worsens flooding downstream, and helps fuel climate change by causing the UK’s biggest carbon store to go up in smoke.

“But the government’s efforts to get wealthy grouse moor owners to give up this archaic practice voluntarily have clearly not worked. The evidence we’ve gathered shows landowners are continuing to set protected moorland ablaze in breach of their own pledges.”

Gove has said that the government will take steps to introduce legislation “if our constructive, voluntary approach does not deliver”.