Two personal assistants to Charles Saatchi and his former wife Nigella Lawson complained they were "being treated worse than Filipino slaves" when they were confronted over their alleged spending sprees on the couple's credit cards, a court has heard.

Rahul Gajjar, finance director for wealthy art dealer Mr Saatchi, told jurors that Elisabetta Grillo, 41, made the claim after he wrote to her and her sister about their alleged use of the celebrity couple's credit cards to buy £685,000 of luxury goods for themselves.

Mr Gajjar told jurors that, at first, Elisabetta, referred to in court as Lisa, and co-defendant Francesca, 35, were open and apologetic about their spending spree.

But he said that after he wrote each a letter outlining how the sisters would pay off their debt which they were expected to sign, they became "agitated".

"Lisa was against the proposal and I remember a reference to 'We're being treated worse than Filipino slaves'," Mr Gajjar, 44, told jurors at Isleworth Crown Court, London.

It is alleged that, between January 1 2008 and December 31 2012, the Grillos, both of Kensington Gardens Square, London, committed fraud by abusing their positions as personal assistants by using a company credit card for personal gain. They deny the charge.

Mr Gajjar said Mr Saatchi and Ms Lawson, who divorced earlier this year, agreed the sisters should continue working for them when the alleged offences came to light early last year.

He told jurors he suggested they continue on their current salaries, with Francesca, who is alleged to have spent the largest amount on herself - £580,000 - expected to pay back £1000 a month, and Elisabetta £250 per month. They would also be able to live rent-free at a flat owned by Ms Lawson in Battersea, London, the court heard.

But Mr Gajjar said the defendants reacted angrily to this. "They were absolutely in disagreement," he said. "They felt it tied them to the company for the rest of their lives."

The court heard a series of text messages exchanged by Mr Gajjar and Francesca, including one where she complained her membership at private club Soho House had been cancelled.

"If they carry on doing stuff like this, I won't have any choice but to go to court," she wrote. In another she said: "If one more small thing happens before we meet, they leave me no choice but going legal."

The court heard that both defendants were given credit cards in Mr Saatchi's Conarco Partnership account with Coutts bank to buy items for the household.

Mr Gajjar told jurors he found out about the alleged fraud after noticing Francesca's average monthly spend on her card was £48,000 while Elisabetta's was £28,000.

In comparison, the other personal assistants employed by Mr Saatchi had a maximum average spend of £8000, while the average amount Ms Lawson herself spent on her card per month was £7000.

Asked by prosecutor Jane Carpenter why he did not go to Mr Saatchi and Ms Lawson with his suspicions straight away, Mr Gajjar said: "It was on the back burner. We were dealing with more corporate matters - more serious corporate matters - and Mr Saatchi and Ms Lawson normally did not have time for what we thought was trivial matters."

While Francesca's credit limit was £25,000 in June 2008, it went up to £50,000 in February 2010, and rose to £100,000 in June 2011.

Jurors heard Francesca ran up a £64,000 bill in June last year as she went on luxury holidays and bought designer clothes.

Mr Gajjar said the defendant made dozens of payments for personal use, including more than £4700 on flights to New York, where she spent £1850 at high-end fashion store Miu Miu and more than £2000 on hotels.

She also used the card to pay a £1280 bill at the Ritz hotel in Paris on the same day she bought a Chloe dress for £723. On June 12 last year she spent £5385 at Miu Miu London, and £2650 at designer store Prada.

Mr Saatchi arrived at the court but shortly afterwards was told his evidence would not be reached. He is expected to testify today.