The two-year extension will see the energy supplier extend its sponsorship until the end of the 2011 series against India.
Npower, which has sponsored home test matches since 2001, says it has also agreed the option for a further two until the end of the next Ashes series in England in 2013.
Despite extending one deal, the energy provider has ended its partner sponsorship of the Twenty20 Cup, a competition it has sponsored since its inception in 2003.
As part of the title sponsorship deal, Npower will have the right to use England players and English and Wales Cricket Board imagery in its marketing as well as advertising at Test Match grounds.
It will also continue grassroots initiative, ‘Npower Urban Cricket’, which it runs in partnership with the ECB.
The news comes a month after the ECB signed a deal with Brit Insurance to replace Vodafone as primary sponsor of the English cricket team.
The ECB is still in negotiations with Natwest over extending the bank’s title sponsorship of domestic one-day cricket.
The final match of the 2009 Ashes series is currently taking place at the Brit Oval in London with the series poised at 1.1.