The research, which was commissioned by mail watchdog Postwatch and conducted by Ipsos Mori, showed that 43% of consumers thought the quality of Royal Mail's service had worsened over the period.
Millie Banerjee, Postwatch's chair, said that the survey's results were unsurprising: "Many deliveries are made later in the day than they used to be.
"Some customers will consider this to be a worse service, even if on average during the two years, more first-class mail was delivered next day than previously. It is important that customer perception is dealt with alongside the hard figures of Royal Mail's performance."
The results of the survey also showed that 42% of consumers thought the quality of the postal service had not changed, while 14% believed it was better.
Banerjee said: "Adam Crozier has acknowledged that Royal Mail needs to become more customer-focused. It certainly needs to engage with customers to show it really does care about every item of mail arriving safely and on time.
"It needs to start to talk up mail as a product. These findings should give Royal Mail food for thought. Royal Mail must both be and be seen to be confident, customer-focused and efficient. Gloomy statements about the poor state of the postal industry damage customer confidence and perceptions and do nothing to improve Royal Mail's position."
Ipsos Mori conducted the research by telephone between July 2 and 6. A representative sample of 2,036 adults in across all age and socio-economic groups in England, Wales and Scotland were contacted.