Daniel Lister, leader of the Conservative opposition, said: “Reform publicly declared a boycott of the training yet many of their councillors still attended.
“That shows they cannot be trusted to stand by their own words.”
Arnull had said in May that his group would follow the party’s national policy of not attending any equality or climate change training.
The council ran equality, diversity and inclusion (EDI) training, but a spokesperson confirmed that back in June, the session was incorporated into a wider session on “lawful decision making” that covered various topics.
“The decision to combine EDI into a broader statutory session was made to consolidate several statutory training topics, recognising the volume of newly elected members and the importance of providing a practical and manageable schedule,” the spokesperson added.
Leader of the Liberal Democrats Jonathan Harris said: “They made a very loud song and dance about not attending, but 25 of them did.
“It’s another example of saying one thing and doing another in my view.”
Reform won 42 seats on the council in May.
Harris also said he was concerned councillors at the session did not have a full understanding of specific climate change legislation.
The administration has attempted to scrap its own net-zero goals.
Labour group leader Sally Keeble also said: “[Reform] were going to boycott equality and diversity training – and then two thirds turned up when it was rebadged under a different name.
“They were going to scrap net-zero – and then scrapped only the local milestones and agreed to do what they could towards national net-zero.
“We need to know what Reform in west Northants stands for and what they will do.”
Arnull told the Local Democracy Reporting Service: “The legal framework training that was attended by Reform councillors and cabinet members covered all statutory decision-making, including their legal duties and responsibilities on local government, housing and planning alongside the Equalities Act.
“It is important that all councillors understand their legal duties and responsibilities so that we can administer a credible and effective Reform council.”