Tougher penalties for extremely violent rapes are to be introduced later this year, the new chairman of the Sentencing Council, Lord Justice Treacy, has revealed.

The appeal court judge this week starts work as head of the body that researches, revises and sets out the underlying tariffs for punishments handed down by magistrates and judges across England and Wales.

Treacy, a member of the Sentencing Council since its establishment in 2010, succeeds Lord Leveson, who chaired the inquiry into phone-hacking and media practices.

The next set of guidelines, affecting 50 individual offences relating to sexual crimes, is due to be published in mid-December. The council has been working on the review for two years.

"Levels of sentencing in sexual offences have gone up in the last 10 years. We will be reflecting that increase in existing judicial practice," Treacy told the Guardian.

"We are not positively encouraging any additional increase because the practice of the courts has already been to recognise the need in certain classes of sexual offences to increase sentences.

"[But] There is a specific example where we have taken a positive decision to increase sentences. It relates to the extreme end of rape where it is associated with very severe violence. We shall be recommending a starting point of 15 years there, whereas previously that sort of level of sentence would have been appropriate to a multiple rape scenario." The guideline sentence for an extremely violent rape currently is a prison term of around 12 years.

The move towards longer prison terms for sex offenders, Treacy believes, is due to the criminal justice system`s greater focus on the experience of victims. "In the last 15 to 20 years … the justice system has recognised the position of the victim. It didn`t do so much in the past. Our guidelines focus much more on what has happened to the victim. The work we do involves substantial consultations with victims` groups so that we understand their view."

Reforms introduced by the Ministry of Justice will soon enable victims to read out their personal impact statements in the courtroom once an offender has been convicted. Treacy wonders how many people will avail themselves of the opportunity, given that a recent survey highlighted popular fears of speaking in public. "It`s not going to change the landscape of sentencing," he said.

Treacy, 64, started as a barrister in 1971, working from chambers in Birmingham. He became a high court judge in 2002 and an appeal court justice last year.

Among notable cases over which he has presided was the trial and conviction last year of Gary Dobson and David Norris for the murder of Stephen Lawrence.

In 2010, he heard the unusual case of the £1.7m robbery of the Menzies World Cargo warehouse at Heathrow – the first significant criminal trial to be heard in England without a jury for more than 400 years.

Chairmanship of the Sentencing Council is a part-time post and Treacy will continue hearing appeal cases. Sentencing guidelines are not always revised upwards, he pointed out. "We brought out guidelines a couple of years ago in relation to drugs mules. People from third world countries are often under economic or greater pressure [to smuggle drugs]. We made a positive recommendation that sentences for them should be reduced. It was felt that people in that category were perhaps being over-sentenced."

Guidelines exist not just to improve the consistency of justice across the country but also so that they can be more readily understood by the public.

"I don`t think judges feel particularly constrained by the guidelines,"

Treacy said. "They are guidelines, not tramlines. The law is that a judge must follow the guidelines unless he or she considers it would not be in the interest of justice to do so."

Judges are free to depart from the guidelines but if their reasons are not good enough their sentences may be varied by the appeal court.

"My experience is that judges are comfortable with the guidelines. The facts of every case are different." Before being introduced, new guidelines are "road-tested" in mock trials to see if any snags or contradictions emerge.

The Sentencing Council`s future programme of reports covers a range of criminal topics.

"Early next year we will be bringing out a guideline on environmental offences. That started because we were approached about the problem of people dropping things such as mattresses in laybys." It will also deal with other pollution offences and the dumping of toxic waste.

"After that we are bringing out guidelines on corporate fraud offences that may be deal with by means of `deferred prosecution agreements`" – a new type of plea bargaining system to speed up complex commercial cases.

Subsequent reports will look at theft offences, ranging from pickpocketing to fraud by employees from their firms, and the effect of guilty pleas on prison terms.

law@guardian