Nervous investors dumped shares in Apple last night on concerns over weakening profit margins and persistent rumours about the health of the technology company's founder, Steve Jobs.

Apple's quarterly profits soared 31% to $1.07bn as customers snapped up record numbers of Mac computers, iPods and iPhones. But the California company provided cautious guidance to Wall Street on its prospects for the rest of the year and offered little to quell speculation surrounding Jobs, whose gaunt appearance drew comment at a recent industry conference.

When asked about Jobs' health during a call with analysts last night, Apple's chief financial officer, Peter Oppenheimer, said: "Steve loves Apple. He serves as CEO at the pleasure of Apple's board and has no plans to leave Apple. Steve's health is a private matter."

In unofficial trading after the close of the stockmarket, Apple's shares slumped 11% to $148.

Jobs, 53, is considered the driving force behind the company's consistent innovation. He was diagnosed with pancreatic cancer five years ago but has recovered.

When Jobs' apparent weight loss was noted last month, Apple said he was taking antibiotics for a minor "bug". The New York Post revived the issue yesterday by quoting unnamed industry and financial sources expressing concern.

Investors have long been worried about the lack of an obvious succession plan at Apple. The issue surfaced last year when Jobs' position was briefly threatened by a scandal over improper pricing of executive share options.

Questions about the entrepreneur proved a distraction from second-quarter figures which revealed Mac sales had rocketed 41% year on year to 2.5m computers, and iPod sales were 12% higher to 11m globally.

Apple sold 717,000 of its touch-screen iPhones during the three months to June. But the company said margins were likely to weaken because of a series of factors including a "back to school" promotion and an unspecified product launch which is being kept strictly under wraps.

The technology company opened 16 Apple stores during the period and this week unveiled its first outlet in Beijing.