The army are on standby to keep Britain's borders secure and handle passport queues during Wednesday's strikes over public service pensions, Francis Maude has confirmed.

The Cabinet Office minister also told unions that they have until the end of the year to accept the government's current offer or it will be taken off the table – the first time that ministers have set a deadline for talks to be concluded.

"We have said there needs to be agreement on the main elements by the end of this year, and if there isn't, we absolutely reserve the right to take those elements off the table," Maude told Sky News.

Details of the military's duties were being discussed by the UK Borders Agency, but Maude said he expected soldiers to man the passport queues, admitting it would not be a great image for Britain to project.

Details and contingency arrangements for the strikes will be discussed on Monday at a special session of the government's emergency committee (Cobra). The strikes have been called in protest at government moves to increase pension contributions by public sector workers.

Ministers acknowledge that two thirds of schools will close and non-emergency operations in the NHS will be disrupted. It is thought that in England around 60,000 non-urgent operations, outpatient appointments, tests and followup appointments will be postponed.

Maude accused the unions of being disingenuous by claiming serious negotiations over the public sector deal had stopped, arguing that further talks were due to take place on Tuesday.

A statement from the Cabinet Office said: "It is simply not true to suggest this big offer, which is fair and affordable to other taxpayers, has not been put to union leaders. This offer is as good as it gets – a deal that most people in the private sector can only dream of being offered.

"We have been clear there is flexibility in this offer within the cost ceiling. That is why each scheme and their unions are discussing the design so that it meets their particular needs of their workforce. By going on strike now the unions have simply jumped the gun."

Maude said the situation had strengthened the case for a change in the law to require more than half of a workforce to vote in favour of action before strikes could legally go ahead.

He told the Sky News Murnaghan programme: "We don't want to go down that path. Strike laws have worked pretty well for quite a long period. We have always said they are kept under review – all laws are kept under review. There has been a case made for changing the law but that is not the way we want to go. We want actually to get on with these negotiations.

"Some of the biggest unions didn't even get [a turnout in strike ballots of] 40%. There is a case to be made and the CBI has made a powerful case for changing the law. The biggest unions had very low turnouts – between a quarter and a third of their members even chose to vote and take part.

"I have made the point that if strikes are called on the basis of these very, very low turnouts then the case made by those who want to see a new threshold put in place, that case will be strengthened."

The Conservative chairman, Michael Fallon, also accused the unions of refusing to negotiate: "There has been a very generous revised offer two weeks ago exempting any trade union member from being caught up in the changes if they are within 10 years of retirement and increases in the accrual rate, which is something the unions have been asking for. It's an improved offer which we put out there and we haven't yet had a proper response."

Mary Bousted, general secretary of ATL, the teachers' union, described negotiations with the government as "like boxing in the dark. We're at a situation where in the eleventh month of negotiations they finally put a figure on the table of what they're prepared to pay for public sector pensions. These negotiations have been characterised by disarray on the part of the government, on a complete dislocation between different departments and the Treasury and it's been like boxing in the dark to try to negotiate with them."

Rachel Reeves, the shadow chief secretary, said ministers should be meeting trade union leaders this weekend, and accused Maude of irresponsibility for failing to do so.

Ed Balls, the shadow chancellor, said he was sympathetic to the strikers, but also said the union negotiators needed to make further concessions.

Brendan Barber, the TUC general secretary, said there was little or no prospect of the strikes being averted.