One of Britain’s most senior prosecutors has warned that “a lot of guilty people” are escaping rape convictions because the rules make it “very, very difficult” to prove cases in court, and also because jurors tend to acquit defendants who present themselves well in the dock.

Ed Beltrami, one of London’s two chief crown prosecutors, said he could give an “absolute guarantee” that rapists who should have been jailed had instead walked free.

He said the main reason was that the law was “all against you” as a prosecutor and it was not enough to show sex had taken place without a woman’s consent. He added that the attitude of some jurors, that a person who “comes across well” and “looks the part” could not be a rapist, was a further obstacle.

“Most people think it’s a straight- forward thing: sexual intercourse without consent, it’s as simple as that, and if you believe the victim that’s the end of it. Not so,” he said in an interview with the Standard. “With murders and things, the task is normally can you prove that the person has done whatever it is. That’s normally it: did you do it? Rape’s not like that. It’s pretty difficult to convict people in those circumstances. I don’t think the public properly understand the complexities. Sexual offences are very, very difficult and deliberately so. I can guarantee a lot of guilty people have been acquitted. I can give you that absolute guarantee because it’s a very difficult standard of proof, particularly with rape.”

Mr Beltrami’s comments will intensify the debate about the way rape cases are dealt with by the justice system, amid conflicting claims that some innocent people are being charged and complaints from campaigners that too few offenders are being convicted.

They follow controversy over the collapse of a succession of cases in which the initial failure of police or prosecutors to disclose vital social media evidence to defence lawyers led to men wrongly put on trial for rape. That has led to suggestions that other innocent people could be in prison.

At the same time, women’s groups and politicians have expressed concern about the large gap between the number of rapes reported to police and the far smaller number of convictions.

Complaints have also been made about the impact which “rape myths” and stereotyped views of how women and men behave have on the decisions of jurors and police dealing with rape allegations.

Mr Beltrami, given a CBE last year for services to the law, said that although the conviction rate for rape in London had begun to rise again after a sharp fall last year, prosecutors continued to face a tough task. This was because even if jurors could be convinced a victim had not agreed to sex, a prosecution would still not succeed if the defendant could show he had a “reasonable belief” she had given her consent.

Mr Beltrami said that even if jurors could be convinced that a victim had not agreed to sex, a prosecution would still not succeed if the defendant could show that he had a “reasonable belief” that she had given her consent.

“The facts of what’s happened are normally right in front of you. The issue is what’s going on in the head of the defendant,” Mr Beltrami said.

“The complainant says I didn’t consent, but the hardest bit is did you [the defendant] reasonably believe that she consented?

“You could have a mistaken, but reasonable belief that she consented – you come along and say whatever she says I believed that in those circumstances she was consenting.

“The prosecution has to prove otherwise - to prove to the jury beyond reasonable doubt when he says that he didn’t really think that at all.

“Most people don’t realise that. You can completely believe the victim, believe that she did not consent, and you get over that hurdle and then the bigger hurdle faces you immediately and it’s a difficult thing to prove. What’s going on in the guy’s head?

“Did he actually believe that she was consenting at the time and if the jury think that he might have done, they’ve got to acquit, because you’ve got to prove it beyond reasonable doubt. So it’s all against you.”

Mr Beltrami said that jurors’ flawed perceptions of defendants and victims could also lead to mistaken verdicts.

“If the defendant comes across well, he looks the part, sometimes people are going to say he’s not going to commit a rape, that can’t be right.

“Alternatively, some defendants .. get rattled and they start come across all aggressive, they think that’s the very sort of person, I can well imagine that he would have raped this person.

“That’s the danger of the system. If you are going to have a trial by your peers .. the jury are going to use their own experience of life in assessing whether they believe someone or not. Some of the techniques that they use can be misleading.”

Mr Beltrami said that despite such failings, he believed the current system was the best available. It was also important to ensure that the law protected against wrongful conviction.

He added: “When people say the conviction rate is low, maybe they should have a thought for where the law is and how difficult it is when those are the rules we play by.

“I’m not necessarily saying they are wrong: it would be a terrible thing for someone to be wrongly convicted of rape. It’s one of the worst offences because you are suddenly labelled a rapist. It’s ruined your life.

“We’ve got to be absolutely sure that when people have been convicted that it’s right to do so and the law is very strict on it, rightly so, but don’t complain that the conviction rate isn’t very high because that’s the price you pay.”

Rape convictions in London fell by 27 per cent in the 12 months to the end of March, to 364. But 16,241 rape cases were recorded in the capital over the two years to the end of October.