UNION bosses have warned a steel firm’s plants will be crippled by strike action unless management relent on changes to workers’ pensions.

Unite has demanded last-ditch talks with Tata Steel over proposals to close the British Steel pension scheme (BSPS).

The BSPS had 143,000 members as of late last year, with assets of about £13.6bn.

The union will start balloting members next week, with the move coming after similar action by the Community, GMB, and Ucatt unions.

If staff walk out, they will join the first national dispute in more than 30 years.

Tata, which supports more than a thousand Teesside jobs and operates the Teesside Beam Mill, near Redcar, and a special profiles plant, in Skinningrove, east Cleveland, says it is aware of the ballot.

A spokesman added it has made an offer to the unions and is still open for talks.

However, Paul Reuter, Unite national officer for steel, said the company has not done enough.

He said: “Following months of intensive negotiations, where the unions offered savings to the company of £850m, Tata has decided it is ideologically wrong for employees to be able to retire with the pensions they were originally promised.

“Tata is consulting our members on its proposal to close the pension scheme and to financially penalise workers and their families in retirement.

“Our members have made it clear the proposed changes are totally unacceptable.

“Unless Tata enters into further meaningful negotiations that would preserve the pension scheme, then the first national industrial action in the industry for 30 years would seem inevitable.”

Unite’s ballot will close on Friday, June 5, with the ballots of Community, GMB and Ucatt ending on Friday, May 29.

But, despite the strike threat, Tata says it has made offers to the unions and remains available for talks.

A spokesman said: “The company tabled proposals to the national officers of the trade unions whereby the defined benefit (final salary) scheme would remain open to future accruals subject to a number of modifications.

“As agreement with the trade unions on these proposals could not be reached, the company has initiated a 60-day statutory consultation period with scheme members on a proposal to close the scheme to future accruals for existing members.

“The consultation process provides opportunities for employees to comment on the proposals and to suggest alternatives that they wish the company to consider, other than the proposal of closure to future accrual.

“The company also remains committed to providing employees with competitive future pension provision.

“We have made clear to the unions our door remains open.”

He added the result of any vote will have no impact on Tata’s ongoing talks with US billionaire Gary Klesch, who is looking to buy its loss-making Long Products division, which includes the Teesside Beam Mill.

Last week, Tata announced it had written-down the value of assets, including part of its North-East business, by £500m.

The action relates to the impairment of goodwill - an intangible asset which includes the value of brands, patents and customer loyalty - on Long Products, which includes the Teesside Beam Mill, at Lackenby, near Redcar, and the Skinningrove special profiles plant, in east Cleveland, where it employs a combined 700 workers.

A decision on the sell-off deal to Mr Klesch is expected in the coming weeks.

Tata is due to announce its financial results for the 2014/2015 year later today