Honda has confirmed it will close its Swindon factory in 2021, ending any hopes that the jobs of 3,500 employees can be saved.

The Japanese carmaker first announced plans to shut the plant in February and efforts by the government and Unite trade union, including a march by hundreds of employees through Swindon, failed to save the plant. Honda said on Monday it would start preparing redundancy packages for employees and finalising the timetable to shut down the plant in Wiltshire.

A Honda spokeswoman said consultants hired by the company had proposed severaloptions to save the plant but the carmaker had decided none of them were viable.

A spokesperson for Theresa May said the closure decision was “clearly disappointing”.

Honda blamed the closure on the need to focus company resources on large electric car markets outside the UK. However, industry experts – and a briefing given by Honda last year – suggested that Brexit played an important role in the decision.

Des Quinn, the national officer at Unite, which represents workers at the plant, said the union had presented an alternative plan which was “likely to be backed by the government with public money”. He said: “Instead we have this body blow which is nothing short of a betrayal of the workforce, customers and the wider supply chain which relies on Honda Swindon for work.”

Facebook Twitter Pinterest A march in Swindon against the closure of the factory. Photograph: Will Oliver/EPA

Hopes had been raised that a buyer could be found, following reports this year that BMW was eyeing the plant. However, the German carmaker denied it had been considering a purchase.

The closure will mean the end of nearly three decades of Honda production in Swindon and could have damaging effects on the town’s economy. Honda’s estimates suggested that at least 3,500 more jobs at suppliers were dependent on the plant, beyond the 3,500 directly employed.

Jason Smith, the head of Honda’s UK manufacturing operations, said: “It is with a heavy heart that today we confirm the closure of Honda’s factory in Swindon. We understand the impact this decision has on our associates, suppliers and the wider community. We are committed to continuing to support them throughout the next phases of the consultation process.”

The business secretary, Greg Clark, said he was “clearly disappointed” with the decision. “We will ensure everything is being done to support those affected and to continue Swindon’s record of attracting investment to create highly skilled jobs that offer fulfilling careers for years to come,” he said.