Gordon Brown is to set out plans for reform in the House of Commons
Gordon Brown is set to announce plans to examine a new system of voting MPs to the House of Commons.
Ministers have discussed an alternative vote system to choose MPs to replace the first past the post method, BBC political editor Nick Robinson said.
Mr Brown's plans, which include a legally binding code of conduct for MPs, come after the expenses scandal.
However, it is not clear what changes could be passed into law before Britain votes at a general election.
The proposals to be announced by Mr Brown are also expected to include plans for an independent authority to police MPs expenses.
A Tory spokeswoman said: "The prime minister is trying to change the rules because he thinks he will lose."
She added: "If the prime minister is so concerned about democratic renewal there is an easy way to fix that and that is to have a general election."
One minister told the BBC: "There is a strong feeling in the cabinet that we should have a bold programme of reform. We don't want to end the next year with a whimper."
BBC political editor Nick Robinson said the prime minister's statement will not endorse a change of voting system nor any particular system but it will call for a debate on whether the electoral system should be changed and which new system could be adopted. It will not set out a timetable for any change.
Our correspondent said Mr Brown chaired a meeting of the new Democratic Renewal Council - a group of ministers - which agreed to consider moving towards the so-called alternative vote or AV system in which voters could list their preferences rather than simply voting for one candidate as now.
The new Home Secretary Alan Johnson recently called for a referendum on electoral reform to be held at the same time as the next general election.
However, sources have told the BBC that it is very unlikely that the necessary consultation and legislation could be carried out in that time.
Our correspondent added that AV was not a form of proportional representation.
Mr Brown will say there would have to be a referendum before any change could be made to the voting system.
He will also say ministers will push ahead with a bill to make the House of Lords largely or fully elected.
The prime minister also will unveil plans for an independent body to police MPs' expenses, a legally binding MPs' code of conduct, and plans to strengthen Commons committees.
Ken Ritchie, chief executive of the Electoral Reform Society, said the proposed reforms were "exceptionally weak" and warned that because the alternative vote system was not proportional it could result in "a more distorted Commons than we have at the moment".
ALTERNATIVE VOTE SYSTEM
Votes rank candidates in order of preference
If any candidate reaches 50% on first choice votes, he or she wins
If no-one does, the candidate with fewest votes is disqualified
Second choice of voters who chose the discarded candidate allocated to remaining candidates
Process repeated until a candidate tips 50%
But he said "it means at least that everybody who is elected at least has majority support in their constituency".
Lib Dem MP Danny Alexander said he did not want to pre-empt the prime minister's announcement, but welcomed the prospect of a debate on the voting system.
"The important thing is to get the debate going, to accept that we need to have change to the electoral system, that the current system is unfair," he said.
Scottish National Party constitutional affairs spokesman Pete Wishart said: "Real electoral reform at Westminster is long overdue, but we also need a General Election so that we have a House of Commons that commands trust."
The new cabinet met for the first time after a week of speculation about Mr Brown's future as leader.
It follows disastrous election results and several resignations by ministers.