Provisional figures from the Met Office had suggested 33.2C was reached at Motherwell on June 28 - the highest temperature ever measured in Scotland.
But this has been discounted following evidence that a stationary vehicle with its engine running had been waiting near the relevant weather station.
“We cannot rule-out the potential for contamination of the measurement by this non-weather-related factor,” the Met Office said.
It means the highest-ever temperature in Scotland reverts to the previous record of 32.9C, which was set at Greycrook in the Scottish Borders on August 9 2003.
Dr Mark McCarthy, head of the Met Office’s National Climate Information Centre, said: “At first review the Motherwell record appeared plausible given the wider conditions on the day and was therefore reported as such.
“However for all new records we undertake further careful investigation to ensure that the measurement is robust.
“This investigation includes statistical analysis of the station data, evaluation against neighbouring sites, and in some cases an additional site visit to check for unexpected issues with the instrument enclosure or equipment to ensure the measurement meets our required standards.”
It was during the investigation that the Met Office found evidence of a stationary vehicle “with its engine running” parked “too close to the observing enclosure and the Stevenson screen housing the thermometers during the afternoon of 28 June.”
This cast some doubt on the Motherwell measurement, meaning the Met Office decided not to accept it as an official new record for Scotland.
Other statistics for the month are unaffected - including a mean temperature for the UK of 14.8C, making it provisionally the third warmest June since records began in 1910.
“The observing site at Motherwell makes a very valuable contribution to our observing network,” the Met Office added.
“The rejection of this particular record does not detract from that or from the hard work of those making the observations. We will continue to work closely with our partners at the site, and across our network, to maintain a high-quality climate-observing network for the nation.”