A survey of London commuters suggests that 4.2m Britons store data on their mobiles that could be used in identity theft in the event they are stolen.
Only six in 10 use a password to limit entry into the phones, according to the survey by security firm Credant.
The survey found that 99% of people use their phones for business in some way, despite 26% of them being told not to.
Security experts say that password protection and, where possible, data encryption, is essential.
The advent of smartphones has seen the types of information that pass through handsets proliferate and it is now much more common to store sensitive information and work-related details on handsets.
The survey found that more than a third of respondents use their phones for sending and receiving business emails, with more than three quarters storing business contact details.
Nearly a quarter store customers' information as well.
As a result, Credant say, lost phones could lead to the theft of a personal identity and the ruin of a professional one.
But the storage of increasingly personal information is also on the rise; the survey found that 16% of people store their bank details on their phones and nearly a quarter store PIN numbers and passwords.
Security experts agree that the storage of such crucial details is ill-advised, and advise users to take advantage of the available security features of a phone.
"If you are ever going to store sensitive info on your mobile phone you must ensure it's protected by a good password - or even better a pass-phrase," said Graham Cluley, Senior Technology Consultant for security firm Sophos.
"It shouldn't be a simple word like password or a dictionary word, or something easy to guess if someone knows you," he added.
"If there's the option, you should encrypt the data on your phone as well. If nothing else you don't want someone who steals your phone making phone calls."
Source : http://news.bbc.co.uk