Police officers need clearer guidance on the new lockdown measures as the PM's announcement was too "loose" and open to interpretation, a body representing law enforcement has said.
The Police Federation for England and Wales said the ambiguity of the new measures for England could make an "already challenging" job "impossible".
This would be "grossly unfair on officers", the federation added.
It comes as lockdown fines rise from £60 to £100 in England from Wednesday.
In his speech on Sunday evening, Prime Minister Boris Johnson announced a "conditional plan" to begin lifting England's coronavirus lockdown.
A new slogan was also unveiled, with "stay home" changing to "stay alert".
Scotland and Wales - which have their own powers over lockdown and have not changed the advice to stay at home - rejected No 10's new slogan, while Labour leader Sir Keir Starmer criticised the PM's plan for lacking clarity.
Further guidance on the rules is due to be published later.
John Apter, chairman of the Police Federation, said further detail on the new measures needed to be "clear and unambiguous... explaining what exactly is expected of the public, so that my colleagues can do their level best to police it".
He added: "Police officers will continue to do their best, but their work must be based on crystal clear guidance, not loose rules that are left open to interpretation - because that will be grossly unfair on officers whose job is already challenging."
Mr Apter also noted Mr Johnson's statement came after a week of "mixed messages and the release of some information which, fuelled by media speculation, meant many people acted as though the lockdown had already ended".
He said: "If the message of what is expected of the public is not clear, then it will make the job of policing this legislation almost impossible."
Existing legislation, known as the Health Protection (Coronavirus, Restrictions) (England) Regulations 2020, will be updated from Wednesday to reflect the changes coming into force.
Among the changes will be a change to fines for those breaching the lockdown measures.
The first fine someone receives if police believe they are flouting restrictions on movement will be £100.
However, this will be lowered to £50 if paid within 14 days, according to the Home Office. Fines will double for each repeat offence, up to a maximum of £3,200.
In Scotland, lockdown fines will remain unchanged after the nation's government found no evidence to suggest an increase was required.
People found to be flouting lockdown rules in Scotland will be first fined £30 by police, which rises to £60 if not paid within 28 days. Cumulative fines for repeat offenders are capped at £960.