Although the pay gap narrowed slightly over all in Britain, the slow pace of change and the regression at some workplaces have prompted calls for companies to be clearer about how they intend to do better.

“In some cases, employers will have a wider gap because they have taken on new female junior staff to build their pipeline,” said Sam Smethers, the chief executive of the Fawcett Society, a women’s rights and equality organization. “But unless they can demonstrate that, it is more likely that they’ve failed to make changes.”

The penalties for companies that do not set out a long-term strategy and follow through should be tougher, Ms. Smethers said.

“Government needs to require employers to publish action plans that we can hold them accountable to, with meaningful sanctions in place for those who do not comply,” she said.

That idea has received some backing in Parliament.

“We need to start taking the next steps and ensuring that companies have action plans that are part of the reporting procedures, and that if they do not try to close their gender pay gap that they face additional fines,” Dawn Butler, the shadow secretary of state for women and equalities for the opposition Labour Party, said in the House of Commons on Wednesday.