* Crisis talks for party leaders and threatened Speaker
* Growing calls for Brown and Speaker Martin to step down
* Poll shows 62 percent of Labour backers say Brown must go
LONDON, May 19 Britain's political leaders called emergency talks on Tuesday to thrash out a new system of expenses after a scandal over claims for everything from manure to porn films threatened to descend into constitutional crisis.
Parliament's most senior officer, Speaker Michael Martin, called the hasty meeting in an effort to quieten growing calls for him to resign for his part in the scandal. But political commentators said he would be forced to quit by the weekend.
"Speaker Martin must go," the left-leaning Guardian newspaper said in an editorial, while the right-leaning The Times called "Next Speaker, Please", accusing Martin of an "exhibition of the inadequacy of the current parliament".
Political experts say forcing Martin's resignation would be a constitutional crisis on a par with the abdication of a monarch or a U.S. president's impeachment.
Opposition Conservative leader David Cameron has said that removing the Speaker would not restore the authority of parliament and has called for a general election due by mid-2010 to be held as early as possible.
"This issue has to be settled one way or another, we can't go on like this," Cameron told BBC radio.
Prime Minister Gordon Brown, grappling with the worst recession since World War Two, is certain to resist calls for an early election.
Brown has called for fundamental reform of the expenses system to try to defuse a scandal that has damaged all the main political parties but is hitting his own Labour Party hardest after 12 years in power.
A poll published on Tuesday showed that most members of Brown's own party think he should step down. Six out of 10 of those polled by the independent Labour website LabourList.org said the party must have a new leader before the election.
Alex Smith, editor of LabourList, said it was a further sign of "grassroots dissatisfaction" with Brown, a former finance minister who replaced Tony Blair in June 2007.
"Brown will need to show clear and decisive action if he is to win back support," Smith said
Health Secretary Alan Johnson, a former trade union leader, is the favourite to replace Brown, according to the poll.
Political leaders are keenly aware that all parties are rapidly losing support, and fear a backlash of voters may register their unhappiness with the major parties by voting for fringe parties in next local and European elections on June 4.
A poll for the Daily Telegraph this week showed support for the Conservatives had fallen six points to 39 percent in the past month, with Labour down four on 23.
Support for the smaller parties, such as the anti-European Union UK Independence Party and far-right British National Party, rose by nine points compared with the previous month.
The executive body of Brown's Labour party was also due to meet on Tuesday to decide whether and how to deal with Labour parliamentarians seen as having abused the expenses system.
Paul Kenny, leader of the GMB trade union which has two seat on Labour's executive committee, said any members of parliament who had profited would find themselves "in deep doo-doo" and should face a deselection process by local Labour members.
"If you can't pass the basic test of being reselected by your own party people...what chance do you think you have got of winning that seat or retaining that seat for any particular party?," he told BBC radio. (Editing by Angus MacSwan)