Unite's assistant general secretary for manufacturing, Steve Turner, said the "rumours are disturbing" and numerous workforce will endure a "very anxious weekend".
The new X-Trail could have created hundreds of jobs. The constant uncertainty, the chaotic government.
Colleague Julie Elliott said Brexit had played an "inevitable role" in the decision, adding "none of it is conducive to encouraging business investment in this country".
Bridget Phillipson, the Labour MP for Houghton and Sunderland South, said: "If confirmed, this would represent deeply troubling news for the north-east economy".
Tory Remainer Anna Soubry tweeted that it was "difficult not to be angry" at the reports, adding: "This is Brexit reality".
In October 2016 Nissan confirmed it would build the X-Trail on Wearside.
The Japanese company refused to shed light on the situation.
A Nissan spokeswoman said: "We do not comment on rumour and speculation".
Steve Turner, Unite's assistant general secretary for manufacturing, said: "It is beyond disappointing that our members are hearing about their futures and the holding back of planned investments in Sunderland through media reports and not directly from the company". Production at the site fell 11 percent in 2018.
"I will be doing everything I can to protect the jobs at the Sunderland plant". The X-Trail is produced exclusively in Japan.
That reverses a decision in late 2016 to build the SUV at Nissan's Sunderland plant in northern England, which employs 7,000 workers.
"They have reiterated today their commitment to the United Kingdom by continuing to manufacture in Sunderland the current Qashqai, Leaf and Juke models and the new Qashqai model from 2020".
There had been concerns that Nissan - part-owned by France's Renault - could move production to France in future to avoid any post-Brexit EU tariffs.
The decision is another huge blow to the UK's vehicle industry which has been warning for years about the impact of Brexit uncertainty on top of other problems such as a slump in diesel sales.
Spending was 589m pounds ($771m), the lowest since the global financial crisis, the Society of Motor Manufacturers said in a statement Thursday.
Production of diesel cars was down by 22 per cent to 561,000 a year ago.
The British auto industry has already seen a dramatic drop in investment since the vote, and business leaders have warned of further blows to the sector after the UK's withdrawal.
It comes after Ford confirmed on Friday that it would be cutting around 370 jobs at its Bridgend plant as it restructures its production across Europe and the United Kingdom.
It followed a similar move by Jaguar Land Rover to reduce its 44,000 workforce by 4,500 under plans to make £2.5 billion of cost savings.