BERLIN - German referees will need to have a police certificate of good conduct to be able to work next season, the German football association (DFB) decided on Thursday as a result of an ongoing tax evasion investigation.

Tax authorities in Germany have focused on unnamed referees who are suspected of having failed to pay taxes on their income. Tax officers went through documents at the DFB headquarters and also searched private homes late last month.

"As of next season a police clearance certificate will be a condition for the deployment of all referees and assistants," the DFB said in a statement.

It said tax authorities had contacted 49 active referees and referee assistants, including all 41 Bundesliga and second Bundesliga referees.

Of those 49 questioned, 42 confirmed they were no longer part of the investigation. In seven cases, those involved said they expected "supplementary assessment", the DFB said.

"Those affected consider it unlikely that there will be any conviction for any tax offence," the DFB said.

The head of the DFB referee commission, Herbert Fandel, said he saw no reason not to trust them and would wait for the results of the probe before taking any other action.

"We have no reason to doubt our referees' statements and we will not use the speculation as the starting point for our actions. We will first await the results of the tax authorities before proceeding with an evaluation," Fandel said.