Temperatures are expected to reach nearly double the average for the time of year on Tuesday on what could be the warmest day of 2017 so far.Parts of the UK will be as warm as Spanish hotspots Ibiza, Barcelona and Madrid. In the South East, temperatures could be as high as 18C, the Met Office said, well above the average maximum 10.3C for the region this time of year. Temperatures will have to soar higher than the 18.3C (65F) recorded on February 20 in Northolt, north-west London, and Kew Gardens, south-west London, to become the hottest day of 2017. The average maximum temperature for this time of year, taking in all of the UK, is 8.9C (48.02F) – significantly lower than highs reached on Monday.Murlough in County Down reached 15.9C (60.62F), the Giant’s Causeway hit 15.8C (60.44F), Usk in Monmouthshire enjoyed 15.4C (59.72F), and Saint James’s Park in central London reached 15.3C (59.54F). Towards the end of the week temperatures will fall but remain just above the average maximum highs for mid-March.Forecasters quashed hopes that it could be the hottest March week ever, as thousands of race-goers prepare to descend on Cheltenham.It is thought that it is unlikely that the mercury will reach 25.6C (78F) – the highest-ever March temperature, recorded on March 29 in 1968 in Mepal, Cambridgeshire.Race-goers can expect cloudy skies but will enjoy largely dry conditions. A river cruiser makes its way past Turf Fen Mill along the river Ant at How Hill in NorfolkCredit:PA Want the best of The Telegraph direct to your email and WhatsApp? Sign up to our free twice-daily Front Page newsletter and new audio briefings.