Dominic Johnson has returned as minister for investment in the department of international trade.
Sunak had axed Lord Johnson from that position after just 26 days when he became prime minister in October.
Lord Johnson founded $7bn investment firm Somerset Capital Management with former cabinet minister Rees-Mogg in 2007.
He also served as Conservative vice-chairman from 2016 to 2019 and has donated £250,000 to the party.
Johnson was only handed a seat in the House of Lords by Liz Truss in October so she could give him a ministerial job.
Rees-Mogg was an early supporter of Truss’ bid to succeed Boris Johnson as prime minister.
Emily Thornberry, Labour’s shadow attorney general and former shadow international trade secretary, said on Twitter: “Surely this isn’t true.
“Dominic Johnson has now been re-appointed by Rishi Sunak as a trade minister?
“The same Dominic Johnson who was sacked from the role a month ago, and then told British farmers they should convert their farms into offices? Please tell me this is a joke.”
Thornberry was referring to a report by Politico which said Johnson had argued agricultural businesses should be helped to “transition their buildings” into “offices or whatever it is.”
HuffPost UK has also learned that Johnson has yet to fill in register of members’ interests – the record of anything which could be seen to influence how MPs or peers vote or act.
On October 26, Johnson told the Lords he would complete the register “hopefully by the end of the month”, but has yet to fulfil this pledge.
Lib Dem international trade spokesperson Lord Purvis said: “Clearly this is a matter of public concern and interest.
“Every Minister has a duty to the people of this country to be open, transparent and accountable – Lord Johnson has failed in this regard yet again
“Trust in politicians is already at an all time low. Every incoming minister with the new prime minister must be clear that they have no conflicts of interest, before they represent the country abroad or in the Houses of Parliament.”
Johnson has already represented the UK abroad – without having declared his interests – during a trade mission to India in mid-October.
The department for international trade has been contacted for comment.