"I am concerned the new form of the Senate might be unsuitable and problematic because appointed senators should not have the mandate to impeach elected office holders," he said.

Abhisit said the 35 charter writers should review once again whether senators should be appointed and whether they should have the mandate on impeachment.

"I fear that if appointed senators are entrusted to preside over impeachment proceedings then political parties might try to interfere with the senatorial appointments, just like what happened under the previous government," he said.

He said the Senate's involvement in the impeachment proceedings had proved impractical, arguing for relying on due process to weed out rogue politicians.

The charter writers on Thursday voted to approve revised provisions the new Senate, specifying 76 elected senators, one seat from each province and the capital, and 74 appointed senators, nominated by each professional group.

In the first draft of the charter they opted for an appointed Senate.

Chiang Mai University law dean Somchai Preechasilapkul said he opposed the idea of senatorial appointments.

All 150 senators should be elected by nationwide voting, he said, explaining that with the entire country as a single constituency, it would be difficult for senatorial candidates to depend on vote-buying or the support of a certain party to get elected.

Charter writer Charan Pakdithanakul said the mixture of appointed and elected senators was a new system that might best serve the public under the circumstances.

"Senatorial appointment could pave the way for many professionals, particularly those with physical handicaps, to participate in politics," he said.

He dismissed concerns about rivalry between appointed and elected senators, saying he was confident they would "work well" together.