Union leaders claim they will be able to deliver strike action at one of London's largest trusts despite a management threat to challenge the dispute in the High Court.

Unison balloted staff at University College London Hospitals trust last week in protest at a 160m private finance initiative deal that would involve the transfer of 600 staff to the private sector.

Just 261 staff out of 955 eligible to vote actually did so, with 75 per cent supporting the action. Unison says this gives it a mandate. But UCLH trust has taken legal advice and believes a strike would be unlawful.

It has written to the union urging it to call off the action, arguing that it is supported by just 4 per cent of the trust's 5,200 staff. The trust is also prepared to go to the High Court to halt the action.

In a statement, trust chief executive Peter Reading said unions would be involved in negotiations with the trust and its business partners Health Management Group later this month. 'Strike action now would deny Unison a place at the table and can only disadvantage the staff concerned, ' he said.

Unison steward Candy Udwin said the figures belied the mood among staff and claimed there would undoubtedly be a strike. 'You can see it in the mood at meetings where people are absolutely horrified at the plans which put profit before patients,' she said.

The full business case for the PFI deal to centralise hospital services on one site will be signed in April.

Unison balloted over the trust's alleged failure to guarantee parity of terms and conditions for staff transferred to the private sector. It wants ancillary services to stay in the NHS.