Unite has called on The Pensions Regulator to investigate the transfer of the Visteon scheme to the Pension Protection Fund.
The union is pushing to achieve "pensions justice" for 3000 scheme members who will receive reduced pensions through the PPF.
In March, this year, Visteon UK - which was previously owned by automobile giant Ford - went into administration with nearly 1000 workers at Basildon, Enfield and Belfast losing their jobs.
Unite joint general secretary Tony Woodley said Ford had made "copper-bottomed promises" to workers before they were transferred to Visteon and "we intend to hold them to those promises".
we intend to hold them to those promises
He added: "Unite stood by Visteon workers in the fight for fair redundancy payments when the company went into administration and the union will not abandon Visteon workers fighting for pensions justice.
"Unite is calling for an urgent meeting with the chairman of Ford and has already asked the pensions regulator to investigate whether there was inappropriate behaviour that sees thousands of workers having to go into the Pensions Protection Fund."
Unite believes members should not have to live with reduced pensions "when Ford made firm commitments to protect the workers' terms and conditions on transfer to Visteon".
A TPR spokeswoman said: "We are very aware of this situation but cannot provide comment on individual cases. As with any investigation, we will consider any evidence presented to us and act in a manner which aims to protect member benefits."
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