Britain’s pensions regulator is braced for a battle with BT over its retirement plan, the country’s biggest private sector pension scheme, which could add billions of pounds to a shortfall in savings for tens of thousands of employees and pensioners.

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The Financial Times has learnt that the regulator has raised questions about the assumptions BT has used in valuing the scheme’s liabilities. It has taken the unusual step of retaining an external actuarial adviser for negotiations with BT over those assumptions, a sign it is braced for tough talks.