There was a 20.5 per cent drop last month with 148,000 few visitors from the UK than for January the previous year.
Spanish hotel price reductions of around two per cent have done little to offset the effects of sterling's dive by almost a quarter to near parity with the euro.
Around 16 million tourists flocked from Briton to its bars and beaches in 2007, accounting for almost one in three of all visitors.
But there were around one million fewer last year, the decline starting in September when numbers were down five per cent and drops in subsequent months.
The latest figures released by Spain's ministry for industry, tourism and commerce represent the lowest number of foreigners visiting the country since records began 15 years ago.
A source said: "These are the worst figures we have had since we began taking records of the number of foreign visitors.
"The drop in British visitors is most significant, as they are traditionally bar far our largest market. The fall is due to the worsening economic situation in the UK and the fall in the value of the pound."
It is estimated that Spain lost one million British tourists in 2008 with many heading out of the Euro zone for countries such as Turkey and Egypt as an alternative.
Andalusia, Spain's southern region which includes the Costa del Sol, registered a massive 26.8 per cent drop in the number of January visitors from the UK.
The Canary Islands, traditionally popular with Brits seeking winter sun, registered 47,000 fewer holidaymakers from the UK, a drop of 17.5 per cent on the same month last year.
With the number of American, German, French and Italian tourists also down, the figures are potentially devastating for Spain, where around 11 per cent of the economy is dependent on tourism.