Magic, which last September recruited former Capital drive-time presenter Neil Fox to host its breakfast show, achieved a significant increase in listeners, in contrast to Capital and Chrysalis Radio's Heart FM.
Comparing January-March with the previous quarter, Magic's reach went up from 1.672m to 1.865m, with its share of listening up from 4.9% to 7%.
Heart dropped into second place in London with a share of 5.7%, down from 6%. Its reach fell from 1.763m to 1.696m.
Capital Radio, which three weeks ago replaced its managing and programme directors, took a 5.5% share and its reach fell from 1.802m to 1.72m.
GCap Media's strategy for the station is to increase listening hours among existing listeners, to which end it reduced the number of ads played on Capital. This focus has meant that unlike Magic it has not been advertising to attract new listeners.
Andria Vidler, managing director of Magic, said the station had achieved the goal it set out last year of getting to number one in London.
"In London, there will be fluctuations in market leadership, but this is another great achievement in Magic's long-term plans."
Chrysalis was happy with the performance of its talk station LBC, which grew its market share from 2.5% to 3.2%. BBC London's share held steady at 1.4%.
Emap's Kiss FM was also unchanged on 3.7%, in the last period before the departure of its long-serving breakfast DJ Bam Bam.
Fellow breakfast host Christian O'Connell's first Rajars at Virgin Radio were accompanied by a small boost in the station's share, up from 3.6% to 3.7%.
Xfm, the station he left last year, climbed from 1.5% to 1.7%.
At GCap, the strong performance of urban music station Choice FM provided a welcome counterpart to Capital. Choice's increased its share from 1.9% to 2.5%.
Smooth FM, the Guardian Media Group radio station, did not fare so well, with a drop in share from 1.7% to 1.6%. Its reach dropped from 541,000 to 473,000 and is down by a quarter compared with the 652,000 listeners it had this quarter last year.
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