Spain's left-wing government said on Monday it would modify a landmark law aimed at fighting sexual violence to close a loophole that has let some convicted offenders reduce their sentences.

Since the law came into force in October, around 20 offenders have reportedly been released and 300 others have seen their sentences reduced.

Prime Minister Pedro Sanchez's Socialist party has announced plans to reform the law.

"In the coming days, we will present a draft bill, a meticulous text that will provide a response and a solution to these undesired effects which we obviously don't want to see repeated in the future," said Education Minister Pilar Alegria, who is also party spokeswoman.

"Logically, the best way to specifically address these undesired effects would be to increase the penalties for sexual offenders," she told reporters.

The controversy erupted barely six weeks after the entry into force of the "Only yes means yes" law, which reformed the criminal code in a bid to define all non-consensual sex as rape.

The overall aim of the law was to shift the focus in cases of sexual violence from the victims' resistance to a women's free and clearly expressed consent.

To this end, the charge of sexual abuse was dropped and everything was grouped under sexual assault. The range of penalties was widened to include all possibilities under that single term.

The law effectively reduces the minimum and the maximum punishment in certain specific cases and hundreds have applied to have their sentences revised.

'Consent must remain at the centre'

Over the weekend, reports that the government was mulling changes to the law prompted tensions between the ruling Socialists and their hard-left junior coalition partner Podemos, which has championed the legislation.

The right-wing opposition Popular Party (PP) had quickly moved to offer parliamentary support if the Socialists wanted to push through the changes without Podemos.

But Podemos reacted angrily. Equality Minister Irene Montero warned that such a move would mean the law reverting to its original format and she vowed to do "whatever necessary" to ensure consent was kept at the centre.

Her stance was hammered home by party leader Ione Belarra on Monday morning.

"Consent has to remain at the heart of the criminal code. We can't go back to the evidentiary ordeal of proving we resisted enough or that we hadn't been drinking," tweeted Belarra, the social rights minister.

Socialist ministers insisted the planned changes would merely address the loopholes and would not touch the issue of consent.

"The correction and modification of the law is designed to avoid any undesired outcomes in the future and the issue of consent will remain at the centre of the law against sexual assault so that women avoid enduring the ordeal of proof in court," cabinet minister Felix Bolanos, a close Sanchez aide, told reporters.

Until now, rape victims had needed to prove they were subjected to violence or intimidation. Without that, the offence was considered "sexual abuse" and carried lighter penalties than rape.

With "sexual abuse" dropped from the reformed criminal code and a much wider range of offences grouped under "sexual assault", a broader range of penalties was required to ensure proportionality.

At the weekend, Montero said it was only a "minority" of judges who had applied the law incorrectly.

She said there were similar teething problems with Spain's landmark 2004 domestic violence legislation, the first in Europe, which faced "almost 200 questions" about its legality in the first years after it was passed.