"Analysis has found pro-EU voices are twice as likely to appear on Radio 4"

It was the Brexit Collection that did it. 

A cross-party group of parliamentarians has taken the findings of a detailed News-watch study of that heavily biased Radio 4 collection of programmes and sent a strongly-worded letter of complaint to Tony Hall, alleging "a constant stream of negativity" from the BBC in the wake of the Brexit vote.

(The Daily Mail reports it here, the Daily Express reports it here, Breitbart reports it here, and the Guardian rides to the BBC's defence here. The full News-watch report can be read here.)

Some of the essential stats from the News-watch study of those 31 Radio 4 programmes and features are:

• Only 2 of those 31 items were sympathetic to Brexit, 8 were neutral and the rest biased against Brexit (to varying degrees).
• Only 23% of contributors in the programmes as a whole spoke in favour of Brexit, against 58% in favour of Remain and 19% who gave a neutral or factual commentary.
• Of the 59 politicians to appear, 37 were pro-Remain. 
• Across the board, 41% of the speakers who supported Leave were ‘ordinary’ contributors (i.e. members of the public with no obvious expertise, for example, in vox pops), compared to only 27% of pro-EU guests. In terms of the number of words spoken, members of the public contributed 34% of the Leave total, compared to only 8% on the Remain side.
• Eleven of the 20 ‘ordinary’ speakers who spoke in favour of Brexit were from two locations in Northern England, Thornaby-on-Tees and Wakefield. All of these were from social classes D and E and together they were responsible for 80% of the words spoken by ‘ordinary’ Leave supporters.

Anyone who's read any of our stuff on The Brexit Collection won't be in the least surprised about any of that. Hopefully, however, it will open a lot more eyes to the remarkable way the BBC has been carrying on, especially since the Brexit vote.