For workers aged 22 and over, the hourly rate will increase by 21 pence to £5.73 per hour. Workers aged 18 to 21 will be entitled to a rise of 17 pence, to £4.77 an hour, and the hourly rate for workers aged 16 to 17 will rise by 13 pence to £3.53. Businesses that do not pay the new rates face prosecution and a fine of up to £5,000.

However, the change is likely to come as an unpleasant surprise for many of the smallest firms, with 44% of those employing just four members of staff (or fewer) totally unaware of the pending increase, according to the research.

"These changes come about every year and, although there is an additional cost involved, it is important for small firms to comply with the law," said Nick Palin, the FPB's Director of Finance and Administration. "Those that don't comply risk paying an even higher price. Recent examples show that the Government is more than willing to prosecute firms if they are in breach of the legislation."

David Robertson, the global Chief Executive of Bibby Financial Services, cited the prosecution in 2007 of a nursery owner for breaking minimum wage laws as evidence of the Government taking a harder line. He said the high level of uncertainty among businesses about National Minimum Wage levels, combined with the Government's tougher stance, could lead to many more firms being punished for non-compliance.

"It is clear that small businesses need to be fully aware of the legislation or risk the consequences," said Mr Robertson. "At a time when business costs are escalating rapidly, as the credit crunch continues to bite and fuel costs soar, the rise in the National Minimum Wage could put a significant strain on many small businesses' cash flow. "

The FPB, which is encouraging its members to plan ahead and budget for the annual changes to the minimum wage, is concerned that, according to the survey, knowledge varies widely across regional and industry sectors. For example, in East Anglia, 58% of small-business-owners are not fully aware of the increase. In the North West, 54% of owners are unaware. However, businesses in the West Midlands buck the trend, with 83% claiming they are fully aware of the increases in October.

Agricultural businesses are the least knowledgeable, with 66% of respondents saying they are uncertain about the increase. Hotels and catering firms are the most knowledgeable, with more than three quarters (78%) stating that they are very aware of the planned increase.

"The National Minimum Wage is a good idea, but companies need to ensure that they are complying with the law," said FPB member Tim Rhodes of Skypark Freight Limited in Liverpool. "The Government needs to do more, though, when it is clamping down on those firms that don't pay the minimum wage. It needs to develop one enforcement policy, stick to it and target all firms, not just hand-pick them."