Barclays is being sued for up to £70 million by Guardian Care Homes, a residential care home operator.
Guardian alleges the bank mis-sold it interest rate hedging products that were based on Libor.
Guardian wants to call as witnesses former chief executive Mr Diamond, former finance director Chris Lucas, ex-chief operating officer Jerry del Missier, ex-investment bank boss Rich Ricci and other senior executives who it said appeared to be relevant to Libor.
A judge yesterday refused Barclays' attempts to adjourn the trial, which is due to start in April.
Mr Diamond and Mr del Missier left Barclays last year shortly after the bank agreed to pay a £300m settlement to US and UK authorities due to the alleged manipulation of Libor.
Mr Lucas and Mr Ricci both left the bank earlier this year.
Barclays said that the allegations of mis-selling Libor-linked interest rate hedging products to Guardian Care Homes had no merit and that the addition of a claim based on what happened with Libor did not change its view.
It said Guardian had advisors and financial experience and owed it £70m.