The MTA’s largest union is planning to “cause havoc” to traffic with a rally outside the agency’s Lower Manhattan headquarters at the end of October amid contract negotiations with the agency.

“We got to fill those streets at 2 Broadway, stop traffic and cause havoc,” Transport Workers Union Local 100 President Tony Utano said in an Aug. 29 speech to members that was posted on the union’s website Thursday.

With looming deficits and ballooning overtime costs, the MTA has sought to rein in growing personnel costs embedded in the TWU contract.

The agency has attributed its soaring overtime costs to low worker availability — the average unionized bus and subway employee was absent 54 days of work in 2018 due to vacation days, sick days, holidays and staff training days, The Post revealed.

But union officials rejected management’s Aug. 14 contract offer, which would maintain 2 percent annual pay increases while doubling worker healthcare contributions, cutting back on overtime and slashing vacation days for new workers, and requiring that worker availability improves by at least three days by the end of 2020.

Union leaders are promising to escalate if the MTA doesn’t ease its push.

“Unless the MTA backs off the ludicrous demands… there will be more aggressive actions than a rally,” TWU International president John Samuelsen told The Wall Street Journal.

In a statement, Utano said workers “don’t trust” the MTA’s employee availability stats.

MTA Chairman Pat Foye is “just trying to distract attention from the real problem, which is gross mismanagement at the MTA under his watch,” Utano said.

“Workers are often ‘unavailable’ for work because they’ve been assaulted on the job because MTA fails to adequately protect them,” he added.

MTA spokesman Tim Minton responded, “MTA employees, like other citizens, are free to peacefully and legally demonstrate.

“We would hope any such activities would avoid inconveniencing fellow New Yorkers whom they regularly serve so well. The MTA is committed to bargaining in good faith and reaching a fair agreement with our represented workforce. Beyond that, we will not negotiate in the press.”