The biggest strike in Northern Ireland’s recent history is under way.

An estimated 150,000 public sector workers are taking part in the walkouts over pay.

Northern Ireland Secretary Chris Heaton-Harris has been urged to intervene to release funding to make delayed pay awards in the absence of devolved government.

However, he has refused, insisting the matter is a devolved one.

Public sector workers in Northern Ireland have not received pay uplifts given to counterparts elsewhere in the UK due to the ongoing political impasse at Stormont.

The Government has offered a financial package worth more than £3 billion to accompany the return of a devolved executive in Belfast.

While the package includes money to make the outstanding pay awards, Mr Heaton-Harris has made clear it will not be made available unless Stormont returns.

There has been no functioning powersharing government in Northern Ireland for almost two years due to a DUP boycott of the institutions in protest at post-Brexit trading arrangements.

The DUP has resisted calls to drop its veto and return to Stormont to enable the pay rises to be implemented.

The party has accused the UK Government of “political blackmail” and has insisted the pay issue should be dealt with separately from the impasse over trade.

The strike is set to have a major impact with schools closed, hospitals offering only Christmas Day-level services, public transport cancelled as well as limited gritting of the roads in zero-degree temperatures.

The Department for Infrastructure has urged the public not to travel unless it is “absolutely essential”, saying there will only be limited gritting on a small number of roads including the M1, M2, A1 and A4.

It is regrettable that the NI parties were unable to come together to elect a Speaker and restore the Executive

Parades, rallies and demonstrations are scheduled to take place across the day.

The largest demonstrations will take place at Belfast City Hall, the Guildhall Square in Londonderry, Omagh court house and Enniskillen town hall.

A large policing operation is also in place.

On Wednesday night, trade unions accused Mr Heaton-Harris of having “failed the people of Northern Ireland”.

Irish Congress of Trade Unions (Ictu) assistant general secretary Gerry Murphy said: “The Secretary of State’s strategy has clearly failed and it has failed the people of Northern Ireland.

“The trade union movement have won the popular argument, as witnessed by the support we have seen from women’s groups, the voluntary sector, students’ unions, the 50 chief executives in the public sector, the PSNI Chief Constable and the head of the NICS (Northern Ireland Civil Service) and the public at large.”

Deputy general secretary of the Nipsa union, Patrick Mulholland, warned that the action could escalate if the dispute is not resolved.

“Following the failed attempts to restore the Assembly and the failure of the Secretary of State to move from his position of releasing the necessary funds to end the action, the strike will only be the start of the disruption,” he said.

“The continued disregard for our members’ concerns will leave us with no choice but to escalate our actions.

“The upcoming weeks will inevitably witness intensified industrial action to draw attention to the urgent need for a needs-based budget to underpin our public services and a fair and decent pay rise for all public sector workers.”

In a statement, Mr Heaton-Harris said it was “regrettable” that the Stormont Assembly had not been resurrected to access funds to make the pay awards to public sector workers.

Thursday also marks the deadline in current legislation for the resumption of Stormont, or the Secretary of State is obliged to call a fresh election.

Mr Heaton-Harris has made clear he will extend that deadline and introduce further legislation to ensure continued delivery of public services in Northern Ireland.

“Today’s strike will be disruptive for people across Northern Ireland. I understand the serious concerns that people across Northern Ireland have about the impact this action will have on vital public services,” he said.

“While public sector pay is devolved, the UK Government has offered a fair and generous package worth over £3 billion which would address public sector pay and provides more than £1 billion to stabilise public services.

“This will require ministers being back to work in Stormont so that decisions on governing can be taken in the round.”

Mr Heaton-Harris said he is “deeply disappointed” the funding offer has not been taken up after a last-ditch attempt to revive Stormont on Wednesday failed.

“This package has been on the table since before Christmas and will remain there, available on day one for an incoming Northern Ireland Executive,” he said.

“It is regrettable that the NI parties were unable to come together yesterday to elect a Speaker and restore the Executive.

“The people of Northern Ireland deserve local political leadership from representatives they have elected to govern on their behalf.

“The UK Government has made a significant offer that would help address the challenges facing public sector pay and Northern Ireland’s finances. It is time for the NI parties to take decisions on how they will best serve the interests of the people of Northern Ireland.”