David Cameron will today urge public sector unions to halt planned industrial action and agree that pension reform is necessary.

The prime minister will address the Local Government Association annual conference and insist that present arrangements are “not fair to the taxpayer” - two days before a mass walkout of teachers, lecturers and civil servants.

The government is finalising contingency plans to deal with the biggest outbreak of industrial action since the coalition was formed, with up to 750,000 workers planning a 24-hour walkout on Thursday.

The stoppage, by members of four unions, will go ahead despite two hours of talks yesterday between ministers and union leaders to try to resolve a bitter row over pensions.

With talks due to resume in July, Mr Cameron was reported to have prepared a “robust” but fair message to unions in a speech to the Local Government Association annual conference.

The scale of the disruption will become clearer today as schools decide whether to close and parents learn if they have to make alternative arrangements for their children.

The TUC welcomed moves by the government to meet again in July after ministers said they recognised that the funding basis for the scheme covering local government workers was different.

Public and Commercial Services union leader Mark Serwotka described the talks as a “farce”, accusing the government of having no interest in negotiating on plans to cut pensions, extend the retirement age and increase contributions.

Mary Bousted, leader of the Association of Teachers and Lecturers, said: “We are disappointed, but not particularly surprised that the government has yet again refused to give us the information we need to carry out negotiations about teachers’ and lecturers’ pensions.”

Cabinet Office minister Francis Maude and chief secretary to the Treasury Danny Alexander maintained that the meeting was constructive and expressed disappointment that Thursday’s strike was going ahead because talks were ongoing.

“We can assure the public that we have rigorous contingency plans in place to ensure that their essential services are maintained during the strike.”