The Liberal Democrats have offered to work with Scottish Labour’s new leader to build a “federal UK”.
Anas Sarwar won the top job on Saturday, beating Monica Lennon and taking 57.6% of the vote.
The Scottish Lib Dem leader congratulated the new leader and offering to work in partnership.
Willie Rennie said: “I would like to congratulate Anas Sarwar on his victory and offer my commiserations to Monica Lennon.
“They are both a credit to their party.
“Liberal Democrats have long championed a federal future for the United Kingdom and in recent times there has been an increased recognition from Gordon Brown and others that the governing architecture that underpins the United Kingdom needs reform.
“I am keen to work with Anas and his party to make reform of the UK a reality.
“It is time to fix the foundations on which our house of nations sits.”
The Scottish Greens said Sarwar will immediately be thrust into the centre of a constitutional wrangle.
Sarwar has previously said pro-independence parties should put their views aside and focus on the recovery from the pandemic.
But Greens co-leader Patrick Harvie said: “I welcome Anas Sarwar to his new role, but he faces a divided party.
“Anas has been clear that Scottish Labour would stand in the way of Scots having a say over their future, but this will not play well with around a third of Labour voters who now say they would back independence.”
SNP depute leader Keith Brown also said Sarwar would have a tough time in his new role.
“Mr Sarwar has, perhaps, an impossible job on his hands breathing life into a party with no new ideas, ambition or vision for Scotland,” he said.
Brown added: “For as long as Labour’s dismissive attitude towards independence, and towards Scotland’s right to decide its own future, continues, they have no chance of rejuvenating a party which has already lost nearly all credibility in the eyes of the Scottish people.”
Speaking after his election, Sarwar expressed his desire to change the party, saying “you haven’t had the Scottish Labour Party you deserve”.
He added: “With rising injustice, inequality and division, I’m sorry we haven’t been good enough.
“And I will work day and night to change that, so we can build the country we all need.”