The Ulster Unionist Party may face a bill of more than £30,000 to meet the cost of legal action taken by three rebel MPs last summer, the BBC has learned.
The three rebel MPs won their legal challenge at the High Court
Jeffrey Donaldson, David Burnside and Martin Smyth won a High Court challenge last June against their suspension for resigning the party whip at Westminster.
It is understood the party has been presented with a draft bill for £35,000 to meet their costs, on top of its own legal bills for the case.
A party spokesman said the issue of costs was currently being negotiated by respective solicitors and had not yet been determined.
The three MPs were suspended from the party after resigning the party whip at Westminster.
They did not support the Good Friday Agreement and called for Mr Trimble to change party policy or step down from the post - in response, Mr Trimble said they should quit the party.
The rebel MPs challenged their suspension in the High Court, where it was ruled to be invalid.
In September, the 900-member Ulster Unionist Council voted in favour of Mr Trimble's motion calling for the MPs to again take party instructions in the Westminster parliament.
Mr Trimble is up for re-election at a party meeting in March
Mr Donaldson eventually resigned from the party last December and later joined the anti-Agreement Democratic Unionists.
Meanwhile, four Ulster Unionist constituency associations have been given until 13 February to fully pay their annual levy to party headquarters.
Failure to pay would lead to a reduction in the amount of delegates which Strangford, Lagan Valley, North Antrim and West Belfast could send to the party's annual meeting in March, where Mr Trimble will put himself up for re-election.
A spokesman said that the party treasurer was continually in contact with associations and affiliated bodies regarding payments, and these matters were of a private nature.