Jane Philpott, one of Justin Trudeau's most trusted ministers, announced today she has resigned from cabinet as the Liberal government's crisis over the SNC-Lavalin affair deepens.

"I must abide by my core values, my ethical responsibilities and constitutional obligations," she said in a written statement.

"There can be a cost to acting on one's principles, but there is a bigger cost to abandoning them."

Trudeau later praised Philpott for her diligent work on crucial government files.

Philpott, the MP for Markham-Stouffville, said she has been considering the events that have shaken the federal government in recent weeks and, after "serious reflection," concluded she must quit.

She said the constitutional convention of cabinet solidarity means ministers are expected to defend all cabinet decisions and other ministers publicly, and must speak in support of the government and its policies.

It grieves me to resign from a portfolio where I was at work to deliver an important mandate. I must abide by my core values, my ethical responsibilities, constitutional obligations. There can be a cost to acting on one’s principles, but there is a bigger cost to abandoning them. <a href="https://t.co/EwO5dtdgG6">pic.twitter.com/EwO5dtdgG6</a> —@janephilpott

"Given this convention and the current circumstances, it is untenable for me to continue to serve as a cabinet minister," she wrote.

"Unfortunately, the evidence of efforts by politicians and/or officials to pressure the former attorney general to intervene in the criminal case involving SNC-Lavalin, and the evidence as to the content of those efforts, have raised serious concerns for me. Those concerns have been augmented by the views expressed by my constituents and other Canadians."

Philpott is a close ally of Jody Wilson-Raybould, the former justice minister and attorney general at the centre of the SNC-Lavalin scandal. Wilson-Raybould testified before a Commons committee last week that 11 officials in the Prime Minister's Office and other offices inappropriately pressured her to override a decision to prosecute SNC-Lavalin on bribery charges related to contracts in Libya.

The Quebec-based global engineering and construction firm faces a 10-year ban on federal contracts if it's convicted.

Philpott said the principles that maintain an independent justice system are at stake in the SNC-Lavalin affair. A fundamental doctrine of the rule of law is that the attorney general not be subjected to political pressure or interference regarding the exercise of prosecutorial discretion in criminal cases.

'Lost confidence' in government's response

"Sadly, I have lost confidence in how the government has dealt with this matter and in how it has responded to the issues raised," Philpott said in the statement.​

Speaking at a climate change rally with Liberal supporters in Toronto Monday night, Trudeau addressed Philpott's resignation and praised her for leading "transformational change" in her various files.

Occasionally shouting to be heard over hecklers in the audience attacking his government's pipeline policies, Trudeau said he'd known how Philpott felt for "some time" but was still disappointed by her decision.

"In a democracy like ours, and in a space where we value our diversity so strongly, we're allowed to have disagreements and debate," he said. "We even encourage it."

Prime Minister Justin Trudeau spoke to a Liberal Party rally in Toronto Monday evening 2:46

He said the SNC-Lavalin affair has prompted much scrutiny and discussion of democratic institutions and how staff conduct themselves.

"Concerns of this nature must be taken very seriously, and I can assure you that I am," he said.

Trudeau said he will be listening to the justice committee testimony and expects more information and opinions will emerge. In the meantime, he encouraged people to focus on the "bigger picture" of the issues Canada faces. He left the rally without taking questions.

A statement from the PMO earlier Monday said Public Service and Procurement Minister Carla Qualtrough will immediately take over her duties as acting president of the Treasury Board.

To the incomparable <a href="https://twitter.com/janephilpott?ref_src=twsrc%5Etfw">@janephilpott</a>, truly the <a href="https://twitter.com/hashtag/MOC?src=hash&ref_src=twsrc%5Etfw">#MOC</a>...For almost 4 years our country has witnessed your constant & unassailable commitment to always doing what is right & best for Cdns. You are a leader of vision & strength & I look forward to continuing to work alongside you. ❤️U. <a href="https://t.co/bwL6ouQjSA">pic.twitter.com/bwL6ouQjSA</a> —@Puglaas

Wilson-Raybould tweeted her response to Philpott's resignation late this afternoon.

"For almost 4 years our country has witnessed your constant & unassailable commitment to always doing what is right & best for Cdns. You are a leader of vision & strength & I look forward to continuing to work alongside you," the Twitter post reads.

Wilson-Raybould resigned from cabinet on Feb. 12, just days after a Feb. 7 Globe and Mail report that said she was pressured as attorney general to overturn the decision to prosecute SNC-Lavalin.

She remains in the Liberal caucus, and confirmed on the weekend that she intends to seek re-election under the Liberal Party banner.

Today, Trudeau said he is still considering whether to allow Wilson-Raybould to remain in the caucus.

Justin Trudeau’s government is in chaos. He wouldn’t allow the former Attorney General to speak freely, so we don’t know her full story. Trudeau’s story changes daily and won’t come clean with Canadians. He needs to fully cooperate with any RCMP investigations and resign. <a href="https://t.co/Qw0yfx9fj9">pic.twitter.com/Qw0yfx9fj9</a> —@AndrewScheer

Conservative Leader Andrew Scheer tweeted that Trudeau's government "is in chaos."

"He wouldn't allow the former attorney general to speak freely, so we don't know her full story. Trudeau's story changes daily and won't come clean with Canadians. He needs to fully cooperate with any RCMP investigations and resign," he said.

During a news conference in Toronto late Monday, Scheer rejected Trudeau's claim that the government was working to protect jobs, insisting that the pressure was imposed for political gain.

Opposition Leader Andrew Scheer spoke to reporters at the Albany Club in Toronto on Monday 2:15

He challenged other Liberal ministers to follow Philpott's example in response to what he called a disgraced prime minister and a government engulfed in scandal.

'Stand up and be heard'

"Is this what you got into politics for, to prop up a prime minister who will subvert the law to win elections and benefit his friends?" he asked. "If not, it's time for them to stand up and be heard like Jane Philpott did today."

Scheer already has called for an RCMP investigation into possible obstruction of justice and has called on Trudeau to resign.

NDP ethics critic Charlie Angus called Philpott's resignation a "watershed moment."

"It is a sad day for Canada to lose a minister with such integrity. Nobody in government has done more to push reconciliation than Ms. Philpott. I have utmost respect for her," he tweeted.

News that <a href="https://twitter.com/janephilpott?ref_src=twsrc%5Etfw">@janephilpott</a> quit cabinet over interference by PMO in SNC prosecution is a watershed moment.<br>It is a sad day for Canada to lose a minister with such integrity. <br>Nobody in government has done more to push reconciliation than Ms. Philpott. <br>I have upmost respect for her. <a href="https://t.co/BeoCbU0AGx">pic.twitter.com/BeoCbU0AGx</a> —@CharlieAngusNDP

After Philpott announced her resignation, she was praised online by Ontario Liberal MP Celina Caesar-Chavannes, who announced Saturday she would not be seeking re-election this year.

Caesar-Chavannes posted on Twitter that she had informed the prime minister and whip of her "tremendously difficult" decision on Feb. 12, but stressed that it was not related to the SNC-Lavalin affair or Wilson-Raybould's testimony at the justice committee.

Praise for Philpott

Today, Caesar-Chavannes tweeted that women in politics can be expected to "make correct decisions, stand for what is right and exit when values are compromised.

"Thank you @janephilpott for articulating this beautifully."

❤️ When you add women, please do not expect the status quo. Expect us to make correct decisions, stand for what is right and exit when values are compromised. Thank you <a href="https://twitter.com/janephilpott?ref_src=twsrc%5Etfw">@janephilpott</a> for articulating this beautifully. +1 <a href="https://t.co/hyVSB4MREk">https://t.co/hyVSB4MREk</a> —@MPCelina

Other cabinet ministers are expressing full confidence in Trudeau. All the ministers responding to queries from CBC News said they support the prime minister and the government.

"My confidence in the Prime Minister remains untouched. I stand by him and believe in his ability to lead a government that delivers for all Canadians," said Labour Minister Patty Hajdu in a statement. "I am sorry to see one of my cabinet colleagues step down and I wish her the best."

On Wednesday, the justice committee will hear testimony from Gerry Butts, Trudeau's close friend and former principal secretary.

Butts to testify Wednesday

Butts resigned from the PMO on Feb. 18 amid allegations he and others exerted political interference on Wilson-Raybould to override the Public Prosecution Service director's decision to prosecute SNC-Lavalin.

"At all times, I and those around me acted with integrity and a singular focus on the best interests of all Canadians," Butts said in a statement at the time.

In her testimony to committee last week, Wilson-Raybould detailed a meeting with Butts on Dec. 5, 2018. She said she wanted to speak about several things, including the barrage of people "hounding" her and her staff about the impact of prosecuting SNC-Lavalin.

"Towards the end of the meeting I raised how I needed everyone to stop talking to me about SNC as I had made up my mind and the engagements were inappropriate," she testified. "Gerry then took over the conversation and said how we need a solution on the SNC stuff — he said I needed to find a solution."

Butts and Privy Council clerk Michael Wernick, who is scheduled to return to the committee Wednesday, are expected to present communications from Wilson-Raybould that they believe casts her version of events in a different light.

With files from the CBC's Katie Simpson