Tony Woodley left Unite in December but is to become an "adviser" to the union.
Conservative MSP Murdo Fraser blasted what he described as the union's "jobs for the boys" culture.
Unite, which is Labour's biggest donor, was formed in 2007 following the merger of Amicus and the Transport and General Workers' Union.
Two general secretaries ran the organisation jointly in the early years: Derek Simpson, on the Amicus side; and Woodley, who had led the T&G.
In 2010, Len McCluskey - a key ally of Woodley - won the election to become the super-union's first sole general secretary.
Simpson received a severance deal worth around £500,000, while Woodley was given the plum job of "executive officer" for organising.
In a letter to members at the time, Woodley noted: "I am pleased to say that I am not jumping ship - I will continue to work for the union until my normal retirement age of 65, in a position I have discussed with Len and about which he will be writing to you separately."
Woodley turned 65 last year and retired before Christmas.
But the veteran trade unionist is set for a comeback as an adviser to the union on the car industry.
Woodley was involved in helping the workforce during the recent industrial dispute with logistics firm DHL, which delivers parts to the car manufacturer Jaguar Land Rover.
A Labour source criticised the advisory role: "Unite members, many of whom are worried about their jobs, will be flabbergasted about this situation. For someone to retire and then come back as an adviser beggars belief."
The retire-rehire practice has attracted controversy in the public and private sectors, as it involves using resources that could be used to hire new blood. The Sunday Herald has revealed numerous cases of quangos, agencies, councils and police bodies giving consultancy work to firms run by former managers and executives.
One Scottish council is reportedly considering putting restrictions on rehiring staff who have taken early retirement or voluntary redundancy.
The revelation comes after Unite was criticised for its tactics last year during a Labour selection contest in Falkirk.
The union recruited more than 100 new members in a bid to help its preferred candidate, Karie Murphy, secure the nomination.
However, it was alleged that some of the new members were signed up without consent, an allegation Unite vigorously denied.
A political row then spiralled into a major industrial dispute, as many of the members were recruited at the Grangemouth oil refinery by shop steward Stevie Deans.
Ineos, which operates the plant, investigated whether Deans had used company time to sign up the members, a probe that nearly led to the site's closure and the loss of hundreds of jobs.
Murdo Fraser said: "Obviously Unite is free to use its members' subscriptions how it sees fit.
"If I was a contributing member, I'd be pretty appalled at this 'jobs for the boys' culture that seems to exist. Perhaps if the union spent less time propping up a failing Labour Party it would be able to make better use of its members' hard-earned donations."
A Unite spokesman said: "Unite is pleased that Tony Woodley has agreed to act in an advisory capacity which will see, from time to time, the union drawing upon his forensic knowledge and expertise of the car industry.
"It is widely acknowledged that Tony has played a central role in securing the future of the UK industry as it continues to go from strength to strength.
"Hugely respected by senior ministers and employers through his work on the automotive council, and with a proven record in securing investment into the UK from global car makers, it makes sense that Unite should continue to draw on his skills as we continue to support this key sector of the UK economy."