The early morning show pulled in 6.9 million listeners in the second quarter of this year down from 7.1 million in the first three months of 2012.
It has lost around half-a-million listeners in the last 12 months, according to the figures revealed by audience measurement body Rajar.
Outspoken Moyles is handing over to Grimshaw after more than eight-and-a-half years fronting the flagship show and his final period at the helm will be covered by the next set of Rajar figures.
Grimshaw - known to listeners as “Grimmy” - has been with the station for five years.
Today’s figures show Radio 2’s Chris Evans easily beat Moyles in the popularity stakes with his breakfast show reaching an audience of 8.9 million, down from 9.2 million in the first three months.
Radio 4’s flagship Today programme pulled in 6.7 million listeners.
The figures show Radio 1’s weekly reach rose slightly to 11.2 million from 11.1 million in the first quarter of this year, while Radio 2 fell from 14.5 million to 14.4 million in the same period.
The audience for Radio 3 increased from 1.9 million to 2 million and Radio 4 also saw a rise from 10.3 million to 10.5 million.
Five Live saw its audience fall from 6.3 million in the first three months of the year to 6.1 million in the second quarter.
Digital-only stations Radio 4 Extra and Radio 1Xtra both pulled in their highest ever audiences with 1.6 million and 1.1 million respectively.
Tim Davie, director BBC audio & music, said: “These figures represent a strong performance across our portfolio and an industry in robust health. Last year we laid out a clear strategy for our digital stations, so it is great to see our digital-only networks breaking records and all four attracting more than a million listeners every week.”
Capital Radio kept the top spot in London with a weekly reach of 2 million, beating Magic 105.4 with 1.9 million and Heart 106.2 with 1.8 million.