Prince William and Harry became stockbrokers for the day to raise money in memory of people who died in the September 11 attacks on New York's twin towers.
But the Duke of Cambridge was accused by his younger brother of being too preoccupied with "baby chat" to focus on the task at hand.
Harry remarked to traders about his brother: "He's on the phone and it's all baby chat."
He then turned to William and said: "Stop flirting, get on with it!"
One broker, Neil Fordham, called William a "natural" on the telephone.
"He even made a bit of a joke," the 45-year-old, from Abridge in Essex, said.
"He said 'It's all a wind-up, I'm Gary'."
The Princes were at brokerage firm BGC Partners in London's Docklands to raise money in memory of people who died in the attacks on New York's Twin Towers on September 11 2001.
BGC lost 658 employees in the attack on the World Trade Centre 12 years ago, and its annual Charity Day, now in its ninth year, has so far raised more than 89 million dollars.
All profits from the trades during the course of the day are donated to good causes, including William's SkillForce and Harry's WellChild organisations.
The Princes each made a handful of high-value deals during their 50-minute visit.
At first they appeared a little timid, but they soon got into the swing of things as the raucous trading floor buzzed around them.
At one point William remarked to a broker: "Bloody hell, was that a billion?!"
After the trader nodded, he turned to Harry to say: "Apparently we should work here."
During one of his deals, Harry urged the trader on the end of the line to put up more money.
"150? Why don't we make that 200?" he said.
The last deal the brothers made was for 25 billion euros - a world record for a forward foreign exchange.
BGC broker Nick Thompson, 45, from Otley, West Yorkshire, mentored Harry through the deal he made alongside William.
He said: "He was great. He's very laid back about it. He did very well.
"Prince Harry was far more confident. Prince William was too busy with the baby chat, rather than the numbers."
After the deal was struck, William remarked: "I'm much better at helicopters."
But executive managing director at BGC Xavier Alcan, who listened in on William's phone call, said he did well.
"He was very pleasant and very courteous," he said.
"He was making enough effort to concentrate on the decision."
Prince Harry and the Duke of Cambridge on the trading floor during the BGC Partners Charity Day in London's Docklands.
Penny Lancaster and Rod Stewart on the trading floor during the BGC Partners Charity Day in London's Docklands.