A senior Labour MSP, Ken Macintosh, has confirmed he will stand for the leadership of the Scottish Labour party when Jim Murphy resigns next month.

Macintosh said he planned to formally announce his candidacy once the leadership contest rules were agreed. Meanwhile, the favourite for the post and current deputy leader, Kezia Dugdale, said she would confirm whether she planned to stand “in the next couple of days”.

Seen as an ally of Murphy, Macintosh holds the Holyrood seat of East Renfrewshire – the same as Murphy’s former constituency in the UK parliament – and came second in the 2011 Scottish leadership contest which was won by Johann Lamont.

Dugdale has already won endorsements from senior Labour figures including Alex Rowley, a former Scottish Labour general secretary and ally of Gordon Brown, and Ian Murray, Labour’s only Scottish MP.

Macintosh has been canvassing colleagues for support but is thought to be confident of his chances of mounting a serious challenge and backs Murphy’s call for Scottish party leaders to be elected using the one-member-one-vote (Omov) system.

In the 2011 contest, Macintosh, regarded as a centre-ground moderate, won a large majority of Labour members’ votes while Lamont won the parliamentary and trade union votes under Scottish Labour’s electoral college system, where each group accounts for 33% of the overall vote.

When he resigns in June, Murphy’s final act will be to urge the party’s executive to scrap that system and replace it with Omov, the system used in UK Labour party elections and within affiliated trade unions.

Speaking after the Holyrood Labour group’s first meeting since Murphy’s resignation announcement on Saturday, Macintosh said: “There’s a lot to go before we actually agree the leadership process and there will be no formal declarations until that point. I think that it is fair to say that I have certainly indicated to some of my colleagues already that I am interested in putting my name forward.

“But at this stage we need to agree a leadership process itself. I think it is fundamental that we move to Omov. I am very much in favour of the union link but the Omov system; if we have got it for the national party, we need it for the local party.”