Prime Minister Andy Burnham has said he did not know the full extent of Russell Brand’s conduct when he publicly supported the comedian in 2014, after Brand was convicted of rape and sexual assault.

Asked on Friday about his earlier social media post, Burnham said: “all I can say is I had no idea of the full extent of what he was doing or what he went on to do”. He described Brand’s crimes as “horrific” and said he had rightly been held to account.

Brand was found guilty on Thursday of one count of rape and three counts of sexual assault. He was acquitted of two other counts of rape and one count of attempted indecent assault. The four-week trial concerned allegations from six women spanning 1999 to 2009.

Some Labour figures sought Brand’s support more than a decade ago, before the police investigation began. Ed Miliband, then the party’s leader, was interviewed by him before the 2015 general election.

Burnham’s own intervention came in 2014, when he was shadow health secretary. Writing on Twitter, he said it was “depressing to see media mission to discredit someone who stops playing their game” and urged Brand to “keep going”.

What precisely prompted that post remains unclear. The previous day, The Sun had alleged that Brand paid rent at a property owned by a landlord in a tax haven while campaigning against tax avoidance. Brand threatened to sue the newspaper and said he would give the money to victims of the Hillsborough disaster, for whom Burnham has long campaigned.

That context distinguishes the public controversy surrounding Burnham’s post from the offences for which Brand has now been convicted. The post itself does not establish what Burnham knew about those offences.

Burnham said: “It's not always the case that people know everything about people in public life, but what is important is that where there is wrongdoing it is exposed.” He praised the victims’ courage and the journalists who exposed Brand’s conduct, describing the convictions as justice for the victims.

Brand, 51, was refused bail and will remain in custody over the weekend. He is due to return to the same court on Monday for sentencing.