Amanda Knox and Raffaele Sollecito had their convictions for the murder of the British student Meredith Kercher reinstated, a dramatic development in the long-running legal saga which paves the way for a possible extradition tussle between Italy and the United States.

The court of appeal in Florence upheld the guilty verdicts for both the 26-year-old American student and her Italian ex-boyfriend at the end of the defendants' four-month-long second appeal. Both have protested their innocence for years.

After nearly 12 hours of deliberations, a packed, silent courtroom heard Judge Alessandro Nencini sentence Knox to 28 years and six months in jail – more than her original conviction – while Sollecito was sentenced to 25 years. Lawyers for both vowed immediately to take the case to Italy's highest appeals court, and their clients appear likely to remain free pending a definitive confirmation which only it can give. Nencini ruled, however, that Sollecito cannot travel out of the country.

Legal observers say Italy is unlikely to request Knox's extradition from the US until and unless the convictions are made final by the court of cassation, a process that could take more than a year.

Standing in court, Kercher's brother Lyle and sister Stephanie looked serious and confused during the lengthy, dense verdict and were briefed on its contents by an official from the British embassy before responding cautiously. "We are still in shock. There's nothing to celebrate," said Kercher's sister Stephanie in the courtroom.

Amanda Knox and Raffaele Sollecito insist they were with each other in his flat at the time of Meredith Kercher's murder. Composite: AP

Her brother said that although the ruling was never going to be "a case of celebrating", it was "the best we could have hoped for". He told journalists the family's feelings were mixed by the knowledge that the sentence still had to be upheld.

"It's hard to sort of feel anything at the moment because we know realistically it's going to go to a further appeal by the defendants, probably some time in spring, so I think we were already prepared for that before this evening's decision," he added.

Asked if they felt any satisfaction at the verdict, he said: "Satisfaction inasmuch as this is what the prosecution's been working for and we've supported what they've been doing throughout. As we've said in the past, no matter what the verdict it was never going to be a case of celebrating anything. That's probably the best we could have hoped for. This is what we've been working towards."

The family's lawyer, Francesca Maresca, said: "I hope this is justice for Meredith and for her family."

The body of Kercher, a 21-year-old Leeds University student from Coulsdon, Surrey, was found in the bedroom of her shared flat in Perugia on the morning of 2 November 2007. She had been in the first weeks of an Erasmus year in the Umbrian hilltop town.

After the verdict, Knox – whose original sentence was for 26 years – issued a fiercely critical statement, attacking the Italian justice system and saying she had "expected better".

"First and foremost it must be recognised that there is no consolation for the Kercher family. Their grief over Meredith's terrible murder will follow them forever. They deserve respect and support," she said, adding she was "frightened and saddened" by "this unjust verdict".

She added: "Having been found innocent before, I expected better from the Italian justice system. The evidence and accusatory theory do not justify a verdict of guilt beyond a reasonable doubt. My family and I have suffered greatly from this wrongful persecution."

Meredith Kercher was found dead in the bedroom of her shared flat in Perugia. Photograph: Kercher family/PA

Sollecito, who had attended the court hearing in the morning but did not return in the afternoon, was said by one of his defence lawyers Luca Maori to have been "crushed" by the news, which he followed on television.

Speaking to the Guardian in an interview filmed in the days before the verdict was announced, Knox said a guilty verdict "would feel like a train wreck". She said she would expect the Italian government to approach the US government with a request for her extradition, "and I don't know what would happen".

She said she hoped the US government would refuse to extradite her. "I'll technically be considered a fugitive. I'm definitely not going back to Italy willingly. They'll have to catch me and pull me back kicking and screaming into a prison that I don't deserve to be in. I will fight for my innocence."

Knox, who was also convicted of slander, said she had hoped for a resolution for both herself and Kercher. "The idea that justice for me automatically means injustice for Meredith horrifies me. Because that is impossible for them to live with, and I hate that idea.

"The only thing I can do is testify to what happened to me and hope that people can take a step back from their emotional investment and try to empathise. You don't have to believe me, but believe that it [wrongful conviction] happens to other people."

She added: "I'm not sitting here gratuitously talking to you about this because I like the attention. The only reason I'm talking about this is because it happens. It can happen to anyone, and it can happen at any time."

Lyle Kercher, left and Stephanie Kercher, centre, talk with their lawyer Francesco Maresca after the judgment. Photograph: Maurizio Degl'Innocenti/EPA

Her words will not have persuaded those who have repeatedly insisted on her guilt, however. In court was Patrick Lumumba, the bar-owner from Perugia whom Knox was definitively convicted last year of slandering.

He said he was not surprised by the sentence. "I was convinced she should be convicted. I was very convinced." Knox served a three-year sentence for slandering Lumumba while in jail for four years from 2007 to 2011. She is appealing against it at the European court of human rights.

The Florence verdict is the latest development in more than six years of legal battles which have seen the former lovers convicted in 2009, then acquitted on appeal in 2011 and then, last year, refused a definitive acquittal by the court of cassation, which ordered a second appeal, accusing the Perugia appeals court which had annulled the convictions of "numerous deficiencies, contradictions and manifest lack of logic".

It was against this backdrop of sharply differing interpretations that the Florence court began hearing the second appeal in late September. There, prosecutors argued that Knox and Sollecito carried out the murder alongside Rudy Guede, a young man from Ivory Coast who was convicted of the killing and is serving a 16-year sentence following a fast-track trial.

Knox and Sollecito, who insist they were with each other in his flat cooking dinner, smoking cannabis and having sex on the night of the murder, vehemently deny this. They say the case against them is a grave miscarriage of justice.

As is standard practice in Italy, the Florence court gave no explanation for its verdict and now has up to 90 days to publish its reasoning.