By Claire Milhench

LONDON (Reuters) - A Goldman Sachs executive footed the bill for prostitutes and the bank paid for a lavish trip to Dubai for the brother of a decision-maker at Libya's sovereign wealth fund, a lawyer for the fund alleged on Monday, in a case it has brought against Goldman in London's High Court.

The Libyan Investment Authority (LIA) is attempting to claw back $1.2 billion from nine trades it carried out with Goldman Sachs in 2008.

In the suit, the $67 billion fund argues that the U.S. investment bank took advantage of its financial naivety by first gaining its trust, then encouraging it to make risky and ultimately worthless investments in equity derivatives.

Goldman Sachs denies all the allegations. "The claims are without merit and we will continue to defend them vigorously," the bank said in an emailed statement to Reuters on Friday.

The LIA's claim hinges in part on its allegations that the trades were procured under "undue influence". It specifically cites an internship that Goldman Sachs provided for Haitem Zarti, the brother of Mustafa Zarti, the LIA's former deputy chief and a key decision-maker at the fund.

The LIA alleges that Mustafa Zarti's willingness to do business with Goldman was influenced by the favorable treatment it was conferring on his brother in providing an internship. Neither side disputes that the internship took place.

In his opening statement on Monday for the LIA, lawyer Roger Masefield said that in February 2008 Goldman Sachs sales team executive Youssef Kabbaj had flown Haitem Zarti from Morocco to Dubai at Goldman Sachs' expense.

According to the LIA's claim, accommodation at the five-star Ritz Carlton was also paid for by Goldman, whilst Kabbaj arranged for two prostitutes to spend the evening with them at a cost of $600, Masefield said.

The bank does not deny that it paid for some flights and hotels, but Kabbaj did not seek to expense the cost of the prostitutes to Goldman, and the bank did not know about it at the time, a source familiar with the bank's position said. Reuters was unable to reach Kabbaj for comment.

Neither Zarti is connected with the fund now. Reuters could not reach either of the brothers for comment.

Legal representatives for Goldman Sachs have not yet had a chance to make their opening statement setting out the bank's defense.

But in the bank's court filings outlining its defense, Goldman Sachs argues that it was always clear the relationship was one in which the LIA and the bank were dealing with each other as commercial counterparties, and from which each side would seek to profit financially.

Goldman Sachs maintains that its relationship with the LIA was at all "material times an arm's length one" between banker and client, and that the trades in question "were not hard for the LIA to understand".

In its court filings, the bank also rebuts the suggestion that the LIA's employees were naive or unsophisticated. "The LIA's plea that it was 'financially illiterate' is as unfounded as it is extreme," the bank argues.

The LIA is also pursuing the French investment bank Societe Generale <SOGN.PA> for some $2.1 billion in relation to another set of trades entered into between 2007 and 2009. SocGen is contesting the case, which is only expected to come to trial in January 2017.

The Goldman Sachs case is scheduled to run for seven weeks.

(Reporting by Claire Milhench; Editing by Kevin Liffey)