Some Liberal MPs are mounting a new effort to oust Justin Trudeau

Pressure is building on the prime minister and his office as a growing number of anxious Liberal MPs are coordinating efforts to force Justin Trudeau to step down as Liberal Party leader, multiple sources have told CBC News.

Disgruntled Liberal MPs have held a series of meetings to discuss a path forward for the party since the surprising Toronto-St. Paul's byelection loss in June.

Those talks accelerated with Parliament's return and the Montreal byelection loss. They escalated further this week with the prime minister and his chief of staff, Katie Telford, out of the country for a summit in Asia.

Some MPs are being asked to sign their names to what amounts to a pledge to stand together in calling for Trudeau to resign, multiple sources said.

The document isn't a letter to be circulated; sources describe it as a vehicle to secure a commitment from MPs to seek a leadership change, and to bind the MPs to that goal if the prime minister and his supporters push back.

MPs were pulled into meetings across Parliament Hill this week to sign on to the demand for change at the top.

Multiple sources told CBC News that MPs have been signing a single document that is being tightly controlled — no copies or photos of it are being circulated.

All the sources spoke to CBC News on the condition they not be named due to the sensitivity of the internal discussions.

These sources said at least 20 MPs have signed the document so far, with others voicing their support for the cause.

One MP who signed the document told CBC News the number of MP signatures has spilled onto a second page.

That same MP said other caucus members have called since word of the document's existence started to spread, indicating they wanted to sign it.

One Liberal who is part of the effort said the intention was to keep this initiative quiet until they had strength in numbers.

The fear is that moving too early would allow the Prime Minister's Office (PMO) to push back and splinter the calls for change, as they did in the summer in the wake of that bruising Toronto byelection defeat.

That's why MPs are being asked to sign the document now. It's an insurance policy of sorts, signed in ink by the MPs' themselves — so that it can't be disavowed if the pressure gets too high.