The head of the National Probation Service has banned his chief officers from lobbying against government policy, according to a leaked letter.
Tories want an inquiry into how the police were used to lobby MPs
Roger Hill's letter was sent on 7 November, when chief police officers were lobbying MPs over anti-terror measures, backed by the home secretary.
Shadow home secretary David Davis said the letter showed the government had "outrageous double standards".
The National Association of Probation Officers called it "extraordinary".
Mr Davis said: "This demonstrates the outrageous double standards of Tony Blair's government.
'Politicising the police'
"On the one hand they want to stop chief probation officers expressing their honest opinions and on the other they encourage chief constables to try to influence the outcome of a parliamentary debate in their favour. They can't have it both ways."
Fellow Tory Stephen Dorrell has accused ministers of "politicising the police".
The leaked letter was sent by Mr Hill, director of the National Probation Service, to chief officers, chairs and regional managers.
Dated 7 November, it says: "I am writing to you formally to remind you of your responsibilities in your roles as statutory office holders, ministerial appointees and civil servants.
"You should not engage in lobbying activity, you must not promulgate misinformation (eg contestability is privatisation) and you should avoid any action that might suggest that you are encouraging staff to lobby against government policy."
It advises that trade unions can respond to issues "as they see fit".
But it says: "As the leaders of the organisation, the employers and the employer body, we have a wholly different responsibility and must discharge it appropriately."
Finally, Mr Hill adds: "The legitimate way to express your views is by responding to the consultation and I continue to expect and encourage you to do that."
The letter was sent out at the same time as Metropolitan Police chief Sir Ian Blair was backing the government's case to extend the detention of terror suspects from 14 days to 90 days without charge.
Assistant Commissioner Andy Hayman also joined Home Secretary Charles Clarke when he tried to change the minds of Labour backbenchers opposed to the move.
But those pleas fell on deaf ears and MPs voted against the plans, causing the first government defeat in eight years.
'Politicising the police'?
The involvement of Sir Ian and other police force chiefs in lobbying MPs angered Conservative MPs.
Senior Tories Stephen Dorrell and Peter Lilley tabled a parliamentary motion condemning ministers for "embroiling them in politics".
"The government asked chief police constables to lobby their local MPs in a way that I think runs the risk of politicising the police," said Mr Dorrell.
Mr Lilley said: "Every chief constable knows their job is up for re-selection in the next year or so. That puts great pressure on them."
The motion has been signed by 11 other Conservative MPs, two DUP MPs, Angus McNeil of the SNP and Lib Dem John Hemming.
Home Office Minister Hazel Blears denied the police had been playing politics by lobbying for 90 day detention, saying they had only being providing their expert advice.
But the appearance of a letter from the probation service director telling staff not to get involved in lobbying activities against government policy is certain to reignite the row.
Mr Fletcher, from the National Association of Probation Officers, said: "The government has to make up its mind whether crown appointees can lobby or they can't - they can't have it both ways."
He said he would like chief officers of probation having the same lobbying powers as the police.