A new Scottish Labour leader can fill the gap between the “extreme” unionist and independence positions which dominate the country’s politics, former first minister Lord McConnell has said.

He insisted Richard Leonard, who resigned as Scottish Labour leader on Thursday, is a “really decent guy” but his leadership “wasn’t connecting in a way that was going to turn the fortunes of the party”.

The former first minister argued there is now a “huge, yawning opportunity” for the Scottish Labour Party to occupy the centre ground of Scottish politics that is divided on the issue of independence.

Lord McConnell, Scottish Labour’s longest-serving leader who led the party for more than five years, acknowledged whoever succeeds Mr Leonard faces a “long-term project”, with the party currently languishing as the third biggest at Holyrood.

He told BBC Radio Scotland’s Good Morning Scotland programme on Friday: “Scotland desperately needs a strong social-democratic force that is primarily interested in the welfare of the people, the strength of our economy and the effectiveness of public services.

“That is missing in Scottish politics at the moment and there’s a huge, yawning gap there that the Scottish Labour Party can fill with leadership, ideas, vision and ambition.

“We need to start to rebuild our organisation, rebuild the confidence, our unity, but more, much more importantly, put the policies in place that inspire people to support us and shows us as a social-democratic force in Scotland that cares about our economy, cares about public services and cares about our society – more than it cares about a polarised constitutional debate.

“I think that is something that could inspire the people of Scotland again and it will do if the leader gets it right.”

Looking ahead to the leadership contest, he advised members to pick a candidate “who wants to be first minister, not just leader of the Labour Party”.

He continued: “The Labour Party, while it has a huge challenge on its hands rebuilding support in Scotland, also has a massive opportunity.

“Because of the division and the extremes that are there between, on the one side, extreme nationalism and, on the other side, the Conservatives who I think are exposed as extreme unionists.

“There’s a really big gap in the middle there where Labour can be the party of policy and about vision for the country and the economy of the country and the services inside the country and our society.

“A strong Labour leader, with a strong party with a clear mission and good policies can actually fill that gap.”