Supporters of the pro-Jeremy Corbyn grassroots movement Momentum are calling on Labour’s national executive committee to discipline two MPs for disloyalty in an attempt to tighten the left’s grip on the party, the Observer can reveal.

In a sign of a power struggle between Corbyn’s backers and those on the centre and right of the party, Momentum members are urging Corbyn loyalists to sign up to demands for action against Frank Field and Simon Danczuk, as well as against a former parliamentary candidate, Emily Benn. The move is part of a counterattack against efforts to expel Corbyn’s leftwing policy adviser Andrew Fisher.

The NEC is already scheduled to discuss further action against Fisher, who has been suspended by general secretary Iain McNicol.

Momentum supporters are circulating documents urging Corbyn loyalists to contact McNicol before the NEC meeting to demand action against the three. A document from Momentum supporters in Southampton, posted on Facebook, has similar wording to documents being circulated among constituency parties across the country.

Its says that Field, a highly respected former minister and now chair of the House of Commons work and pensions select committee, should be disciplined for saying that any Labour MP deselected as a result of leftwing purge should stand again as an independent. It argues that Danczuk is guilty of “serial disloyalty” against Corbyn and says Benn, granddaughter of Tony Benn, has previously showed support for the Women’s Equality party.

Senior Labour sources said it was clear Momentum supporters wanted to protect Fisher in his job by highlighting what they saw as comparable offences by others. Benn, the Labour candidate for Croydon south in May’s general election, made a formal complaint against Fisher last month, saying he had supported Class War rather than her campaign before the May election. Danczuk has said he might be prepared to stand as a stalking horse challenger next year. The NEC may also debate whether to change rules on potential leadership challenges, which would make it more difficult to remove Corbyn from the job.

A spokesman for Momentum said the organisation did not want anyone to be expelled and that the campaigns were not official.