Cherie Blair, the wife of the former Labour Prime Minister Tony Blair, voted for the Liberal Democrats in the European elections, ITV News understands.
Sources close to Mrs Blair, who is still a Labour member, told ITV News she "did not vote" for the party and was "happy for it to be known".
Earlier this week, Mr Blair's former Director of Communications Alastair Campbell was expelled from the Labour Party after admitting he voted for the Lib Dems.
A number of high profile Labour figures have since publicly declared they also voted for another parties because of Jeremy Corbyn's ambiguous position on Brexit, including former ministers Charles Clarke, Bob Ainsworth and Betty Boothroyd.
Labour's shadow Attorney General Shami Chakrabarti said the decision to kick Mr Campbell out the party would be re-examined.
Mr Campbell said: "I'm still in the Labour Party, as far as I'm concerned, and I'll always be Labour.
"This is the single biggest issue facing the country, Brexit, and the Labour party was hammered in these elections because of the incoherent position that was taken."
Mr Corbyn's office said they would not be commenting on individual cases.
The Labour leader is under pressure to reinstate Mr Campbell, with deputy leader Tom Watson calling the expulsion "spiteful".