In a landmark ruling, the judges concluded that Ms Jones could be reinstated as MP for Newark because of her successful appeal against conviction for expenses fraud.
Lord Justice Kennedy, sitting with Mr Justice Mitchell, delivered their judgment in response to a request for clarification of electoral law from John Morris QC, the Attorney General, on behalf of the Speaker of the Commons.
To the cheers of Labour MPs, Ms Jones took her seat in the Commons chamber after the Speaker, Betty Boothroyd, declared that the Newark seat was no longer vacant and a by-election would not be held.
Ms Jones ended decades of Tory rule in the Nottinghamshire seat when she gained a majority of just over 3,000 at the last general election. Labour strategists had initially feared that a by-election triggered by allegations of "sleaze" would offer the Tories a chance to score a much- needed victory. However, at the end of William Hague's worst fortnight since he became the party leader, even Conservative MPs expressed relief that the by-election would not be held.
Ms Jones, 42, was forced to vacate the seat and ordered to complete 100 hours of community service after she was convicted of election expenses fraud at Nottingham Crown Court last month. The conviction was quashed earlier this month when the High Court ruled that the jury had been misdirected on the scope of the 1983 Representation of the People Act.
Ms Jones and Labour Party officials expected she would be immediately reinstated by the Commons authorities in response to the verdict. However, the Speaker said that as the situation was "unprecedented" it was up to the courts rather than Parliament to rule on reinstatement.
There is no provision in the 1983 Act for reinstatement after a quashed conviction, although Ms Jones's lawyers said that "natural justice" demanded she retake her seat. The key legal issue was whether a by-election should automatically follow the declaration of a seat as vacant, even if the conviction that caused the vacancy had been overruled. Philip Sales, counsel for Mr Morris, had told the court that "ordinary fairness" would suggest that she should be reinstated.
Outside the court, Ms Jones said she was pleased, but also relieved that the affair was finally over. "It has been a very stressful time for me and my family. I am very grateful to all the people who have written to me, who have never lost faith," she said. "I was very heartened by that. There has been very little feeling of negativity in Newark. But I am looking forward to getting back to resuming my seat.".
A spokeswoman for the Labour Party said: "We're very pleased that Fiona Jones has been reinstated following a difficult and uncertain time for her and the people she was elected to represent. With her name now totally cleared, Fiona can get back to work for the people of Newark."
A Tory party spokeswoman said: "We accepted the earlier court ruling and we also respect today's judgment."
Fraser Kemp, Labour MP for Houghton and Washington East and the party's campaigns manager for the Midlands, said part of him would have relished a by-election after recent turmoil in the Tory party.
Mr Kemp said that the whole affair had proved the need for the Home Office to work with a new electoral commission to clarify confusing laws, which had been designed for Victorian times rather than the 21st century.
Ms Jones, who arrived in the Commons chamber flanked by the government whip Graham Allen, made her first comments during a debate on development on green-belt land.
To cheers, she said: "Can I just take a very welcome opportunity on behalf of my constituents in Newark to say that I welcome the proposals that the Government are putting forward, and I would like to say that all my constituents, I know, welcome the fact that the Government will build on brownfield sites instead of greenfield sites."
Nick Raynsford, Environment minister, said: "I'm delighted to know that at the first opportunity [Ms Jones] is back here speaking for her constituents."
The full, detailed reasons for the High Court's decision in Ms Jones's favour will be revealed in open court today.Reuse content