Negotiations over Bristol's new arena have been called a "damning saga and a sham" by opposition councillors.
The 12,000-seat venue was first mooted in 2003, but has been beset by delays and rising costs.
Opposition councillors have now said reports showing talks taking place over one of two proposed sites as early as spring 2017 show "bias".
But the city council said it was "normal activities for these kinds of proposals".
Two locations are being considered for the arena, one by Buckingham Group on council-owned land near Temple Meads station and the other at an airport hangar in Filton, five miles away.
In June, a value-for-money assessment, commissioned by Bristol City Council, found the Temple Island scheme would cost £33m more than the approved budget of £123m and that building at the former Filton airfield site instead would be cheaper.
But a Freedom of Information request, first reported by the Bristol Post, has now revealed discussions over the Filton site took place between its Malaysian owners YTL and the council as early as spring 2017.
Other documents show officers helping YTL to draft a letter to the mayor, as well as discussions on how to overcome planning hurdles.
Opposition councillors have described the detailed discussions between council officers and YTL, several months before the company's bid became public, as showing bias.
Conservative councillor Geoff Gollop said the emails "made it clear" the council and mayor were working with YTL "on a regular basis, and with KPMG who were doing the value for money report".
He said: "The proposed operator for Temple Meads had no approach from KPMG... so what we are seeing is a biased value for money report."
But a council spokesman said meetings and providing information "are normal activities for these kinds of proposals", especially "with implications for around £150m of public money", adding: "It shouldn't be mistaken for showing a specific proposal favour."
Earlier this week, the city's former mayor George Ferguson criticised incumbent Marvin Rees of falling "hook, line and sinker" for the hangar proposals during a trip to Malaysia in December, hosted by YTL.
A spokesperson for YTL said: "As major investors in the city region, YTL have maintained contact with both the former mayor, George Ferguson, and the current mayor, Marvin Rees, and with senior politicians and officers.
"More recently there has been specific contact to provide information around the possible development of an arena in the hangars at Filton. These emails reflect that."