High street chaos is predicted today as shoppers who use credit and debit cards will be forced to tap in their personal identity numbers, or pins.
It is estimated that about one in ten tills has still be switched, the group behind the chip and pin system said.
These include scores of B&Q stores as well as smaller shops and restaurants.
Businesses without chip and pin are liable for credit card fraud carried out on their premises.
Until today, shoppers have been able to sign credit or debit card slips instead of tapping in their pins.
Anyone not happy with the new system will have to apply to their providers for a special chip and pin signature card.
But Nick Goulding of the Forum of Private Businesess said now the use of pins was compulsory, problems could also arise for shoppers who forget their numbers.
"This has all the hallmarks of a shambles. There is a real risk that large numbers of shoppers will be turned away at tills if they cannot remember their pin," he said.
DIY chain, B&Q, said its chip and pin roll-out would not be complete until April because all 329 outlets have had to be upgraded.
The Association of Payment Clearing Services (Apacs), the body which represents banks and credit card firms, said 770,000 out of 860,000 tills had so far been changed over.
But some independent shops, medium-sized chains and restaurants don't intend to switch. Apacs spokesman Ben Thomas said: "Some people won't choose to enrol in the system. These will tend to be retailers who do much smaller transactions and where fraud is less likely to occur." He warned: "Many who don't come across will find that fraudsters will find them out as a loophole."
The Association of Convenience Stores, which represents 32,000 shops, said smaller retailers were least likely to switch because of the up-front costs.
ACS spokesman Shane Brennan said: "Chip and pin has had massive success on cutting down on card fraud but there is growing concern amongst our members that it is being displaced to other kinds of crime, like theft against retailers."
Mr Brennan urged independents to upgrade to chip and pin where possible.
The British Hospitality Association, which represents around 20,000 hotels and restaurants, warned initial feedback from restaurants was that customers were leaving fewer tips for staff because not all chip and pin machines give them the option.
The British Retail Consortium, which represents more than 80 per cent of the UK retail market, said most members had the new system up and running.
"People have been gearing up to it for a couple of years now and we've got to the point where we had to put a date on it," the organisation said.
Also yesterday, Apacs' latest research suggested that up to 1.3 per cent of debit card-holders and five per cent of credit cardholders are not using their pins to verify transactions.