Iyiola Omisore has already been in office for two years
An election appeals tribunal in Nigeria has sacked a senator, ruling that his election two years ago was rigged.
A new election must be held within 90 days to replace Iyiola Omisore, who represented part of Osun state in the south-west for the ruling PDP.
The opposition Action Congress had appealed against a tribunal ruling last year which upheld his election in 2007.
It means Mr Omisore must step down from his influential post of chairman of the Senate budget and finance committee.
The election appeals tribunal ruled that there had been widespread breaches of the electoral act, including the use of political thugs to break up voting stations and intimidate voters.
A BBC website reader on election day in 2007 texted from Iwo in Osun state: "Polling booths in Oke-Odo are deserted. People are scared to come out to vote."
After his election to the presidency in 2007, Umaru Yar'Adua promised to reform the electoral process as most observers said it was characterised by vote-rigging.