David Cameron's shake-up of the top jobs in the civil service "may not be sustainable" and risks reducing Whitehall's most senior mandarins to the status of "political courtiers", a cross-party committee of MPs has warned.
The commons public administration committee warned that splitting the roles of cabinet secretary and head of the home civil service "could lead to weaker leadership and disperse power at a critical time of change in government".
Four former cabinet secretaries have expressed "deep reservations" about the changes, said the committee, in a newly-published report which called for a review of the new structure within six months.
The report recommended the appointment of a full-time head of the civil service, who should also be permanent secretary of the cabinet office and should attend cabinet regularly and have the ear of the prime minister.
When Sir Gus O'Donnell retired as Whitehall's most senior official at the end of last year, the prime minister appointed Sir Jeremy Heywood as his successor as cabinet secretary - effectively the top civil servant at 10 Downing Street.
But, in a significant shake-up of responsibilities, Cameron gave Sir Gus's former job as head of the home civil service to Sir Bob Kerslake, who also retained his role as top civil servant at the department of communities and local government (DCLG). Meanwhile, Sir Gus's old post of permanent secretary at the cabinet office went to Ian Watmore.
Cameron told MPs in November that the changes would "modernise" the way government worked, adding: "I absolutely feel this is the right thing to do."
But today's report raised concerns that Sir Bob may be left with "unequal status" to Sir Jeremy and limited access to the prime minister.
And the MPs said they doubted that Sir Bob would be able to fulfil his duties as Head of the Home Civil Service in the two days a week allotted to the role, while running the DCLG in the other three.
The posts of cabinet secretary and head of the civil service were combined in the early 1980s, and a former holder of the positions Lord Wilson described the decision to reverse the merger as "a brave step into the past". Experience from the past suggested it would be "extremely hard to work".
Another former cabinet secretary Lord Turnbull told the committee the move was "a step backwards", recalling the "unhappy" experience of civil service heads before 1983 who found that the job left them "typecast as the shop steward of the mandarinate".
The committee said: "We are concerned that this could lead to weaker leadership and disperse power at a critical time of change in government.
"The civil service needs to have clear and effective leadership and a strong reforming head to make the necessary changes.
"We are disturbed that all four surviving former cabinet secretaries have expressed deep reservations.
"We therefore believe it would be right for the government to monitor these changes and review their effectiveness in due course."
Committee chairman Bernard Jenkin, the Conservative MP for Harwich and North Essex, said: "A prime minister is entitled to reorganise the top of the civil service as he thinks best suits the nature of his administration and the challenges it faces, but civil servants are not merely servants of ministers.
"There must be no impression that the most senior civil servants are becoming political courtiers. They play a vital role in our national life.
"Nor is this the time to divide or dilute civil service leadership. The head of the civil service cannot be reduced to cutting ribbons at job centres on Friday afternoons.