An umpire secretly banned for a year for betting offences by the tennis authorities was immediately allowed to work at some of the world’s biggest tournaments, the Guardian can reveal.

The Croat Denis Pitner, who was suspended in August after regularly logging on to a betting account from which bets were placed on matches, was a line judge at both the US Open, one of tennis's four grand slam events, later the same month and also at an ATP world tour event in Doha last month, featuring Novak Djokovic and Rafael Nadal. A photograph of Pitner sunning himself in Qatar remains on his Facebook page.

The United States Tennis Association, which runs the US Open, said it was "shocked" to be told the news by the Guardian – and blamed the mistake on a flaw in their processes and Pitner being placed on the banned list just one day before the grand slam tournament started on August 25th.

A spokesman told the Guardian: “After learning within the last 24 hours that an official on the ‘do not credential’ list may have worked at the 2015 US Open as a linesman, the USTA immediately investigated the claim. The USTA was shocked to find that this was in fact the case.

“As we have now determined, Denis Pitner had been approved to work the US Open as a linesman on 13 July. The USTA was notified that Mr Pitner was placed on the ‘do not credential’ list on 24 August. Mr Pitner had already picked up his credential prior to the USTA being notified. Due to a flaw in our process, which we are investigating now, Mr Pitner’s credential was not cancelled. For this reason, he did work as a linesman at the 2015 US Open.”

The USTA spokesman confirmed that Pitner had worked in the qualifiers and then the main draw at the US Open through to Thursday September 10th. He also insisted that the mistake would be urgently investigated, adding: “The USTA takes this matter extremely seriously and has made the investigation of what caused the error its highest priority. We will also work with the newly created independent review panel to ensure instances such as this are not repeated in the future.”

The ATP, which runs men’s tennis, blamed a “breach in procedures” for Pitner being allowed to work in Doha. A spokesman confirmed it would also review the issue to ensure it did not happen again.

During his ban Pitner also worked as a chair umpire at the ITF Super Seniors team championships in Croatia, an amateur tournament for players over 70, held under the auspices of the Croatian Tennis Federation in September. An ITF spokeswoman said: "The Croatian Federation was informed in August of his suspension. As with the USTA and ATP, we are looking into ways to ensure that this does not happen again."

This latest news will heap yet more embarrassment on tennis authorities, following a Guardian investigation that earlier this week revealed that four umpires had been secretly suspended for allegedly taking bribes from betting syndicates. Another umpire, Kirill Parfenov from Kazakhstan, was also secretly banned for life in February 2015 for contacting another official on Facebook in an attempt to manipulate the scoring of matches.

In both the Parfenov and Pitner cases, the tennis authorities never publicly released details, alerting only a small number of tournament directors and national tennis federations.

This latest news will also raise fresh questions about transparency levels in tennis. Earlier this week the ITF said that Pitner had been suspended in early August yet it appears that it took around three weeks for the Croat to be put on the banned list, on August 24th. The Guardian has been unable to find an adequate reason for the delay.

It also caps a bad few weeks for the sport, following the BBC and BuzzFeed allegation before the Australian Open that the tennis authorities had not done enough to investigate match fixing in the sport.

Tennis authorities insisted on Friday that the independent review into the integrity of the game – which was announced at the Australian Open last month – would "thoroughly investigate allegations of corruption in international professional tennis and the effectiveness of existing anti-corruption practices and procedures". The International Tennis Federation, the grand slam board and the men's and women's associations also boldly promised to implement all its recommendations.

The review, which is expected to take at least a year, will look into match-fixing allegations as well as the banning of two umpires on the Futures Tour for corruption. It will be undertaken by a three-person panel, led by Adam Lewis QC, which will have the right to demand the production of documents and other information, and be able to consult with law enforcement agencies, betting operators and other relevant organisations.

The statement from the ITF, the ATP, the WTA and the grand slam board said: “The independent review panel will review the effectiveness and appropriateness of the tennis anti-corruption programme, the tennis integrity unit and the tennis integrity protection programme and recommend any suggested changes.”

“The governing bodies of international tennis will publish this document and have committed to fund and implement all of the IRP’s recommendations.”

(Guardian service)