Proposed rules for how judges should deal with sex offenders could result in softer than intended sentences unless they are tightened up, say MPs.
The Commons home affairs committee has responded to draft rules from the Sentencing Guidelines Council (SGC).
The SGC has said rapists should be punished less severely if their victim had been sexually familiar with them immediately before the attack.
But the MPs say that guidance needs to be altered before it comes into force.
The committee says it agrees with the SGC that "relationship rape" is as serious as "stranger rape" and sexual familiarity can be a mitigating factor.
But the MPs say: "We stress that any reduction of sentence should be small, and may not apply in all such circumstances."
The guidelines cover 57 different offences and the MPs have criticised the SGC for not giving them enough time to consider the draft new guidelines fully.
They want the rules altered to make clear that "provocation" or being under the influence of drugs or drink should not be a mitigating factor in rape cases.
The committee says filming and photographing assaults should result in tougher punishments.
And causing "grave psychological damage to children", especially through long-term abuse, should be another aggravating factor - as should buying or selling another person for trafficking.
Committee chairman John Denham said the MPs' review of the draft guidelines had been "severely limited" by the short amount of time they had to consider the plans.
"We regret that the committee has not had sufficient time to consider them in as much detail as we would have liked," he said.
"We believe that our proposed changes would make the guidelines clearer and reduce that chance of an offender wrongly obtaining a softer than intended sentence."