Reform chairman Zia Yusuf has branded Sue Gray the "ultimate swamp creature" following her appointment as a life peer by Sir Keir Starmer.
Speaking on GB News, Yusuf questioned Labour's commitment to reforming the civil service, pointing to Gray's appointment as chief of staff as contradictory to their stated aims.
"How can you possibly claim that you want to take on the bloc, that you want to take on this ridiculously bloated, bureaucratic state and then make Sue Gray your first appointment as chief of staff?" he said.
He added that Gray's appointment undermined Labour's claims about tackling bureaucracy, stating: "If you want to drain the swamp, she is the ultimate swamp creature."
Sue Gray became Starmer's chief of staff last year after a career as a senior civil servant in Whitehall and at Stormont in BelfastGetty
Gray later became Starmer's chief of staff, moving from her civil service position to join the Labour leader's team.
She is among 30 new Labour peers to be appointed to the House of Lords, strengthening the party's representation in the upper chamber.
Gray's tenure as chief of staff ended after less than three months following a brutal power struggle within Sir Keir's Downing Street operation.
The conflict emerged between Gray and Morgan McSweeney, who has since taken over her position.
Zia Yusuf joined Martin Daubney on GB News
GB NEWS
The situation was further complicated by revelations about Gray's £170,000 salary, which exceeded the prime minister's own pay.
The internal tensions had begun almost immediately after Labour's election victory.
Among the other Labour peers appointed are former MPs Thangam Debbonaire, Julie Elliott, Lyn Brown, Steve McCabe and Kevin Brennan.
Former Welsh first minister Carwyn Jones has also been nominated by the PM.
Conservative leader Kemi Badenoch has put forward six nominations, including former deputy prime minister Dame Therese Coffey and ex-housing minister Rachel Maclean.
The Liberal Democrats have made two nominations to the upper chamber.
Business Secretary Jonathan Reynolds, when asked about Gray's potential ministerial return, said decisions about Government were for the prime minister but he would be "really pleased" to see strong people helping deliver the Government's agenda.