Footfall fell by 3.3% last month according to the British Retail Consortium (BRC) and Springboard. That was lower than the 6% decline in March, but was still an "unprecedented" 4.8% decline over the two-month period. Diane Wehrle, of Springboard, said: "Not since the depths of recession in 2009 has footfall over March and April declined to such a degree." "Even then the drop was less severe at minus 3.8%."
New data also showed that the town centre vacancy rate rose to 9.2%, with all areas of the UK, except Greater London, reporting an increase.
- The towns and cities with the highest and lowest wages
BRC chief Helen Dickinson said changing shopping habits and tough trading were weighing on high streets.
As well as "a wet start to April", she said a shift in shopping habits, along with a "highly challenging" business environment, was having a significant impact on high streets: "In April nearly 1 in 10 shops in town centres was vacant."
There were 4,083 new store openings in 2017, the lowest since 2010, but 5,855 outlets closed, meaning a total of 1,772 shops disappeared, according to the Local Data Company.
The BRC and Springboard found that although footfall improved in the second two weeks of the month, people were choosing to spend their money on leisure activities such as going to restaurants or pubs rather than shopping.
Ms Wehrle added: "The parlous state of retail trading is highlighted by the fact that footfall post-5pm recovered in the last two weeks of the month, rising by 5.9%, whilst daytime footfall dropped by 0.1%.
"So it is clear that retail trading is doubly challenged by a thrifty consumer, in concert with a continuing predisposition towards leisure rather than retail spend."
UK wage growth has trailed inflation for more than a year.
Last month, official figures for the three months to February showed that average wages rose by 2.8%, still below the 2.9% inflation rate using the consumer prices index (CPI) measure.
The latest threat to the High Street came on Saturday when it emerged that the owner of Poundworld had put the discount retailer up for sale, the BBC understands.
Private equity firm TPG had been looking at closing about 100 of Poundworlds 355 stores as part of a restructuring plan.
However, that process has been put on hold after receiving expressions of interest in the chain.