The Duke of Sussex and six other high-profile claimants must make a £9.54m interim payment within seven days towards the Daily Mail publisher's legal costs after losing their high court case over allegations of unlawful information gathering, a judge has ruled.

Associated Newspapers Ltd strongly denied the claims made by Prince Harry and co-claimants including Doreen Lawrence and Elton John.

Mr Justice Nicklin dismissed all the claims last month after an 11-week trial in London earlier this year. The court had been told that ANL's legal costs were £34.5m.

Lawyers for the claimants, who also include John's husband, David Furnish, the actors Sadie Frost and Liz Hurley and the former Liberal Democrat minister Simon Hughes, had argued the interim payment should be just over £7.9m. The £9.54m must be paid by 4pm on 28 August.

In the costs ruling on Friday, Nicklin also said that, aside from where orders had already been made, ANL's costs should be determined on an indemnity basis. This means that, as the losing party, the prince and the others must pay towards ANL's costs without the publisher needing to show these were reasonable and proportionate.

An indemnity basis is more unusual than a standard basis, and there must be conduct or circumstances "outside of the norm".

Nicklin said in the ruling that it was "was not simply a case in which serious allegations were made and failed", nor "merely a hard-fought piece of litigation in which, after a lengthy trial, the court preferred the evidence and submissions of one side over the other.

"The claims, and the manner in which they were brought, pleaded, pursued, maintained and publicly advanced, involved a combination of circumstances and conduct which took the litigation outside the ordinary and reasonable conduct of civil proceedings."

He said in the 44-page ruling that he was granting costs on this stricter basis for several reasons, including "the speculative and substantially inferential character of the claims at their origin", the "exceptional breadth" of the cases, "and the public presentation of allegations of serious criminality and impropriety which were not ultimately established".

He said the group were criticised for "the making and maintenance of very serious allegations against many individuals, the failure to reassess those allegations when the evidence required reassessment, and the continued pursuit of allegations which were unsupported or had ceased to be realistically maintainable".

"In my judgment, that conduct was unreasonable to a high degree," he added.

Nicklin refused to cap the costs.

In a statement, ANL said the ruling "is a devastating critique of an attempt to destroy a newspaper and the reputations of its journalists, editors and executives".

It added that despite the "collapse in evidence to support the extremely serious allegations of 'abhorrent criminal activity, crimes and terrible and reprehensible covert acts' by the Mail," the allegations were "not withdrawn, and were pursued until the bitter end of the trial".

"The truth is that these outrageous claims should never have been brought. That they were pursued raises disturbing questions about the conduct of elements of the legal profession."

Hughes said he was "disappointed and surprised" the judge had declined to limit the costs.

The ruling does not mean ANL is entitled to recover all the costs it incurred. Any incurred unreasonably will be disallowed and if the parties cannot agree, the costs will be assessed by a costs judge.

The parties have been given until 2 October to lodge permission to appeal in respect of the trial judgment or the costs ruling.

Friday's ruling means the claimants may have to pay millions out of their own pockets to cover the costs, with the court having previously been told there was a shortfall between ANL's total costs and the group's insurance cover of a combined £16.2m.

The judge described the £34m costs claim from ANL as "excessive", "striking" and "largely unexplained", but said imposing a ceiling "would risk unfairness and be vulnerable to the charge that it was arbitrary".

The court did not make any findings that any of the claimants had acted dishonestly.

David Bailey-Vella, the chair of the Association of Costs Lawyers, said: "This could not have gone much worse for Prince Harry and the other claimants. The only chink of light is the judge's comments about the £34m costs claim appearing 'excessive' and his 'real concerns' about how much of it would meet that test before a costs judge."

The judgment comes after it was made public that Harry and Meghan are moving back to the UK this month. According to the Australian news website News.com.au, Meghan is said to have been offered a role in an upcoming project based in the UK, though no further details have been confirmed. A source close to the duchess did not deny the report, according to the BBC. The former Suits star quit acting when she and Harry got married in 2018.