Deputy Chief Cabinet Secretary Yoshitada Konoike, who is also married, denied having an affair with the woman, but said she had visited him on several occasions at his government housing compound.

Mr Konoike, an upper house lawmaker from Prime Minister Taro Aso's ruling Liberal Democratic Party, also denied giving the unnamed woman a key, but said he was at fault for causing what he called a misunderstanding.

Mr Aso, who is under mounting pressure to call a snap election after his popularity rating sagged to around 20 per cent, played down the incident.

'It is not our concern as it is a personal matter,' he told reporters. 'I cannot comment as I don't have accurate information.'

But Chief Cabinet Secretary Takeo Kawamura, said Mr Konoike had been given a warning.

'Although it is a private matter, it is extremely regrettable that such an article has been published,' Mr Kawamura told reporters of the article in Thursday's edition of Shukan Shincho magazine.

'I have given him a warning because it is not appropriate for a person in the inner circle of government to be misunderstood.'

The chief cabinet secretary acts as a key advisor to the prime minister and as the government's top spokesperson. -- AFP