* UDI chief Lagarde says Fillon will seal right's failure
* Fillon party leaders to meet Monday for crisis talks
* Mass rally planned on Sunday in Paris to support Fillon
* Election graphic tmsnrt.rs/2jLwO20
By John Irish
PARIS, March 5 (Reuters) - The head of France's centre-right UDI party said on Sunday he wanted presidential candidate Francois Fillon to quit the election race in favour of former prime minister Alain Juppe warning that if Fillon continued defeat was certain.
Once the frontrunner, Fillon is mired in a scandal over his wife's pay, and his campaign has been in serious trouble since he learned last week that he could be placed under formal investigation for misuse of public funds.
He is under growing pressure as party leaders prepare a crisis meeting for Monday to discuss the situation ahead of a March 17 deadline when all presidential candidates must be formally endorsed by at least 500 elected officials.
After a string of resignations among advisers and backers, the 63-year old former conservative prime minister is banking on a rally of supporters in Paris on Sunday to show his detractors that he remains their best hope to win the presidency.
"With Fillon it's a certain failure. This (rally) is an excess because you don't put the street up against the (state's) institutions," Jean-Christophe Lagarde told Europe 1 radio. "Even if there are 200,000 people, to win a presidential election you need 20 million people."
His UDI party, which represents between 2 to 5 percent of voting intentions, on Friday said it was withdrawing its support for the former prime minister.
Fillon pulled out of an early Monday morning radio appearance that aimed to discuss his campaign, the show's host said on Sunday. "Francois Fillon has officially cancelled his appearance on Europe 1's morning programme," presenter Thomas Sotto said on Twitter.
There was no immediate comment from Fillon's camp.
Opinion polls continue to show Fillon would fail to make the second round of the April/May election. Instead, centrist Emmanuel Macron is consolidating his position as favourite to win a second-round head-to-head against far-right National Front candidate Marine Le Pen.
Fillon has denied any wrongdoing and complained of judicial and media bias that amounted to a "political assassination".
His attack on the judiciary in particular has caused unease within his ranks and Sunday's demonstration has worried some on the right that it will be hijacked by hardline conservative movements.
His backers hope to get at least 45,000 people at today's rally to show he still carries favour among grassroots supporters.
In her first public remarks since the fake job allegations surfaced, Fillon's wife Penelope, told the Journal du Dimanche that her work activities had been real and insisted that her husband should go all the way.
"He needed someone to carry out his tasks. If it hadn't been me, he would have paid someone else to do it, so we decided that it would be me," she said.
But an Ifop poll of 1,002 people published on Saturday showed that more than 70 percent of French voters want him to drop out. Support from his camp has also fallen to 53 percent from 70 percent two weeks ago.
The same survey also suggested Alain Juppe, who lost to Fillon in the November party primary, was the best placed to step in. A poll on Friday showed that Juppe would win the April 23 first round, although the current mayor of Bordeaux has until now ruled out a comeback.
"In the Olympics when the gold medal winner is disqualified then it's the silver medal holder that takes over," Lagarde said.
Additional reporting by Simon Carraud; Editing by Dominic Evans