Northern Ireland is set to be brought to a standstill next week in the biggest strike in recent history, trade unions have said.

Teachers, nurses, health workers, education support workers, police staff and civil servants are among those who are to take part in a generalised day of action on January 18 over an outstanding pay award for public sector workers.

The 14 trade unions involved have a combined membership of more than 150,000 workers.

On Wednesday, the three public transport unions – Unite, GMB and Siptu – confirmed their members will join the action.

They said a public transport strike will start at staggered times from midnight next Thursday.

It will involve all bus and rail drivers, engineers, signal workers and conciliation staff.

“It will bring all public transport services in Northern Ireland to a standstill,” the unions said.

They have urged Northern Ireland Secretary Chris Heaton-Harris to intervene in the absence of a functioning devolved government and make the pay award.

Sinn Fein vice president Michelle O’Neill said public sector workers have been “forced into a horrible position” of taking strike action.

During talks between Mr Heaton-Harris and the Stormont parties in December, a £3.3 billion financial package to accompany the return of devolution was offered.

The package would include money to make an outstanding pay award to public sector workers.

But with Stormont remaining effectively collapsed, that funding has not materialised.

An NIO spokesperson previously said the package will remain on the table until a new executive is formed to allocate it.

PSNI Temporary Assistant Chief Constable Melanie Jones said planning is under way to “mitigate any risk in critical functions in the event of staff absences as a result of industrial action”.

“I want to reassure the public that we will have the resources in place to allow us to keep people safe,” she said.

Unite general secretary Sharon Graham said it will be an “unprecedented demonstration of the power of workers in Northern Ireland”.

“For almost two years, the absence of a functioning Stormont executive has led to growing paralysis and crisis across public sector services,” she said.

“This situation has been made worse by a Secretary of State for Northern Ireland who has sought to leverage the crisis in the current negotiations through imposing a punishing budgetary settlement.

“Unite will not allow the livelihoods of workers and the funding of public services to be used as a bargaining chip by this government.”

GMB senior organiser Alan Perry said its members “have simply had enough”.

“They will not be used as political pawns in a process that would see the return of the Northern Ireland Executive,” he said.

“The Secretary of State made it clear that money is there; he must do the honourable thing and make it available now to address the concerns of members who desperately need it.”

Speaking to reporters at Belfast City Hall, Ms O’Neill said it is “not acceptable” that workers are not paid fairly.

She said: “It’s not tolerable and it’s not the position that they should be in. So we fully support them in their action.”

“We have had years of under-investment in this place, just before Christmas the British Government have finally recognised that we have been underfunded.

“What we need to see now is a restored executive and all of the parties working together. We take the financial offer and we want to add to that financial contribution because the Treasury have failed public services here.”

She added: “Public sector workers are waiting anxiously to know if they are going to get money into their pockets which they absolutely deserve. We support that.”