Unite union members based at plants in Cowley, Goodwood, near Chichester, Hams Hall in the West Midlands and Swindon, turned down the offer by 56.6%.
Workers have already staged four walkouts during the dispute, which is over the closure of a final-salary pension scheme.
BMW said the offer was "fair" and it was disappointed at the ballot result.
Unite representatives will meet on Tuesday to discuss the next steps in the dispute, which has seen the first-ever strikes by the company's British employees.
Three 24-hour strikes scheduled in May were suspended while workers considered the offer, which the union did not recommend to its members.
Unite national officer Fred Hanna said: "It is clear that it did not go far enough or deal with the concerns many of our members have over BMW's pension plans.
"We would urge BMW bosses to reflect on the result and listen to the workforce by further engaging in meaningful talks with Unite."
A BMW spokesman said: "We are now considering the implications of the ballot result and we will be meeting with the union in due course to discuss next steps.
"We believe the offer, which resulted from lengthy negotiations with the union since September last year, was fair.
"It was designed to improve competitiveness, which is in the long-term interests of all our employees in the future."

Sierra Leone and Guinea both recorded the lowest weekly total of confirmed Ebola cases since August.
Liberia, which reported no new cases on two days last week, had its lowest weekly total since June.
The death toll from the world's worst Ebola outbreak has reached 8,429 with 21,296 cases so far.
According to the latest World Health Organization (WHO) report, Guinea, Liberia and Sierra Leone now all have sufficient capacity to bury all the people known to have died from Ebola.
But it said under-reporting of deaths meant that not all burials were being done safely.
While cases were decreasing in Sierra Leone, it remained the worst-affected country, with western areas still reporting the most new transmissions, the WHO said.
Last week, there were 59 new reported cases in the capital, Freetown.
The spirit of Freetown seems to be back with residents in the capital celebrating, albeit quietly, the low Ebola infection figures. For over a week the reported daily infections have been in single figures and on Tuesday there were only two deaths.
Like Premier League football fans, people converge at internet cafes and street corners to follow the daily updates. The usually grim faces are now grinning. The ubiquitous blaring of ambulance sirens has also reduced considerably. In fact, the national daily infection rate is around 20 - in November it was 60.
"Ebola is in trouble," says Ahmed Turay, a teenager who has not been to classes since July. "I look forward to returning to school," he says, smiling broadly.
The eastern district of Kailahun, which first recorded Ebola eight months ago, has had no cases for 35 days and the other former hotspot of Kenema has had only four cases since November. But the authorities are warning all to exercise caution, insisting Ebola is still here.
Meanwhile, in Guinea it has been announced that schools and universities are to reopen next Monday after a five-month closure because of Ebola.
The BBC's Alhassan Sillah in Guinea says the authorities there are also stepping up Ebola information campaigns as in some areas people are still suspicious of official attempts to fight the disease.
Over the weekend, two policemen were killed by villagers who feared they had brought Ebola to the western district of Forecariah.
Earlier this month, the outgoing head of the UN team fighting Ebola, Anthony Banbury, said he believed cases of the virus would be brought down to zero by the end of 2015.
Ebola basics: What you need to know

Sussex Police said the emergency services were called to the flat in Sutherland Avenue, Bexhill, at 13:35 BST.
The victim died from her injuries at the scene.
Following a police search, a 21-year-old man from Bexhill was arrested on suspicion of murder and is currently in custody. The suspect was known to the victim, police said.
Workers at car giant BMW have voted to reject an offer aimed at resolving a long-running dispute over pensions.
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New Ebola cases in the three West African countries worst affected by the deadly outbreak of the virus are declining, weekly UN figures show.
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A woman has died after being stabbed in a flat in East Sussex.