UKIP took a "deliberate, informed and calculated" decision to ensure that a defamation action brought by Rotherham's three Labour MPs should not be settled before the General Election, the High Court has ruled.
A year ago, Mr Justice Warby said that Jane Collins, MEP for Yorkshire and North Lincolnshire, should pay £54,000 in damages to each MP over remarks she made about the town's child abuse scandal.
She was also directed to pay their costs with an interim payment of £120,000.
Sir Kevin Barron, MP for Rother Valley, John Healey, who represents Wentworth and Dearne, and Sarah Champion, MP for Rotherham, sued Ms Collins for libel and slander over a speech she gave at UKIP's conference in September 2014 - a month after a report found about 1,400 children in the area had been abused between 1997 and 2013.
The judge said she alleged that each of them knew many of the details of the exploitation yet deliberately chose not to intervene.
She also expressed the opinion that they acted out of political correctness, political cowardice or political selfishness and were guilty of misconduct so grave that it was or should be criminal, as it aided the perpetrators.
He heard that the allegations were the "talk of the tearoom" in Parliament in the run-up to the election.
Ms Collins refused to withdraw them throughout the campaign, had not apologised and repeatedly tried to delay the case.
She made an offer of amends which was accepted - but the amount of compensation could not be agreed so had to be assessed.
The judge said that all three MPs felt that their careers were at stake and their integrity was under serious attack.
On Thursday, he said that none of the sums had been paid, and it was the MPs' case that UKIP should bear the costs as it had provided financial support to Ms Collins and sought to influence the conduct of the action for its own ends.
UKIP accepted that it had funded Ms Collins in the sum of £31,000 but said its primary concern was to help her settle the claims.
The judge said it would be unjust to make any order for the period up to the end of February 2015 as the party was not to be held responsible for Ms Collins making the speech.
It began by acting in good faith, funding the provision of initial advice and representation for a defendant towards whom it felt some moral responsibility.
Thereafter, it played a supportive role, aiming to facilitate and fund a settlement.
But, things changed significantly in late February and early March 2015, he added.
"In that period, the party took a deliberate, informed and calculated decision, for reasons of party political advantage, to ensure that the case was not settled before the General Election.
"In my judgement, it very probably did thereby prevent a settlement that it had been advised should be made and which would otherwise have occurred quite swiftly."
The likelihood was that, but for its role, the case would have been settled by March 20 2015 at Ukip's expense.
Ms Collins' conduct as a litigant in person between June 2015 and January 2017 was not caused or contributed to by Ukip and it would be unjust to make them pay towards the costs incurred as a result, he said.
But there was every likelihood that a settlement in spring 2015 would have obviated any need for the assessment hearing.
He ordered that Ukip should pay the MPs costs from March 20 2015 to June 23 2015, and costs from the assessment hearing.
The order would be subject to detailed assessment which would give UKIP the opportunity to challenge the sums claimed.
His ruling made the party liable in addition to, and not in substitution for, the orders made against Ms Collins, who remains liable.
A UKIP spokesman said later: "While we are disappointed with the decision it is clear that UKIP are being treated as a party driven by goodwill and we believe that will be reflected in any final decision on costs."
Reacting to the judgement the three MPs, in a joint statement, said: "This judgement confirms UKIP used the unfounded allegations by Jane Collins for political advantage.
"At the highest level UKIP knew Jane Collins' case was 'hopeless' but blocked any settlement in our favour before the 2015 General Election because they believed it would win them votes.
"This deliberate strategy delayed the full vindication we finally won, and hugely increased the legal costs.
"This means UKIP will rightly pay a substantial share of the legal bill.
"Their actions behind the scenes forced the costs to soar and compounded the damage from Jane Collins' unfounded allegations."