The solicitor general has been criticised for asking a court to increase a three-and-a-half-year jail term imposed on a trainee barrister and new mother convicted of falsely accusing her former boyfriend of a series of rapes and assaults.

Robert Buckland QC asked the court of appeal to increase the sentence handed to Rhiannon Brooker, 30, after she was found guilty of fabricating allegations that led to her ex-partner Paul Fensome being jailed for 36 days. But three appeal court judges ruled that the term should not be increased.

The shadow attorney general, Emily Thornberry, said she was surprised the solicitor general had considered the sentence unduly lenient and argued that the government’s priority ought to be ensuring more rapists were successfully prosecuted.

She said: “Ms Brooker, a mother with a young child, received a custodial sentence of over three years for a non-violent offence. The court of appeal considered that there was nothing wrong with the sentence.

“I think the priority of the law officers should be to address the widening gulf between the soaring numbers of rape allegations made to the police and the dwindling proportion that ever get prosecuted.”

At her sentencing hearing at Bristol crown court in June, Brooker’s barrister Sarah Elliott QC argued that she was a vulnerable woman who had a damaged upbringing, but the birth of her daughter with a new partner nine months before had profoundly changed her. She added: “Every single day in prison will be agony.”

Jailing Brooker, the trial judge Julian Lambert said she had acted in an “utterly wicked” way and argued that false claims made it more difficult for real rape victims to be believed in court.

The original sentence was heavily criticised by women’s rights campaigners who claimed such a long jail term would put rape victims off going to the police for fear they could be imprisoned if they were not believed in court.

Before the appeal court hearing, the campaign and support group Women Against Rape argued that an even longer sentence would “strike terror” in women and children who had suffered abuse. Lisa Longstaff, of WAR, said: “It will discourage them from coming forward and boost the die-hard myth that women and children often lie about rape.”

At the appeal court in London on Thursday the solicitor general argued that the original sentence “failed adequately to reflect the aggravating features of the case, particularly the time spent in custody by the victim, and the calculated and repeated nature of the lies told”.

Following the hearing, Buckland said: “I acknowledge the strength of feeling that this case has attracted. I referred this case to the court of appeal because I was of the view that the sentence of three and a half years’ imprisonment was unduly lenient, taking into account all the circumstances, which included a significant degree of planning over a long period. False rape claims seriously damage the public perception of and response to genuine rape allegations.”

He said he accepted the court’s decision that Brooker’s sentence was not unduly lenient and should remain unchanged.

Brooker’s partner, Hamish McKenna, said the attempt to have the sentence increased was “vindictive and nasty”. He said their daughter, now one, was missing her mother desperately. “She is very clingy, she has night terrors, she needs her mother,” he said.

After the hearing, Longstaff said: “We at WAR are very relieved that the court of appeal took into account her being a young mother of a small child and the terrible impact of prison on both of them.”