David Cameron joined calls for the suspension of Labour MP Naz Shah over anti-Semitic social media posts dubbed "offensive and unacceptable" by Jeremy Corbyn.
The Labour leader gave the Bradford West MP a personal reprimand a day after she apologised and quit as an aide to shadow chancellor John McDonnell over her behaviour - before she was elected to the Commons.
But he fell short of taking any disciplinary action, saying he believes Ms Shah "does not hold these views" despite shadow energy secretary Lisa Nandy saying the party faced "real problems" if it was seen not to act.
In a Facebook post in 2014, before she became MP for Bradford West, Ms Shah shared a graphic which showed an image of Israel's outline superimposed onto a map of the US under the headline "Solution for Israel-Palestine Conflict - Relocate Israel into United States", with the comment "problem solved".
The Guido Fawkes website - which published the post - also pointed to another made before Ms Shah was an MP which used the hashtag #IsraelApartheid above a quote saying "Never forget that everything Hitler did in Germany was legal".
Ms Shah wrote in Jewish News that she wished to make an "unequivocal apology for statements and ideas that I have foolishly endorsed in the past".
"The manner and tone of what I wrote in haste is not excusable. With the understanding of the issues I have now I would never have posted them. I have to own up to the fact that ignorance is not a defence."
But the Prime Minister hit out in the Commons over the lack of firmer action against her.
"Anti-Semitism is racism and we should call it out and fight it wherever we see it.
"The fact that we have got a Labour Member of Parliament with the Labour whip who made remarks about the transportation of people from Israel to America and talked about a 'solution' is quite extraordinary."
Pointing to recent comments by Mr McDonnell that people with such views should be excluded - even if they claim to have reformed their views - Mr Cameron added: "Frankly it'll be too many hours in the day before it happens to the MP in question".
"Perhaps if you could deal with anti-Semites in your own party, we'd all be prepared to listen to you a bit more," he said.
Ms Nandy said she had told the leader's office that procedures must be applied "without exception".
She told BBC2's Daily Politics: "I made clear my view to the leader's office that we should suspend anybody who makes anti-Semitic remarks, in line with our policy, and investigate.
"It is right that we look at the circumstances around what has happened. But I think our policy is absolutely clear that we suspend people and then investigate.
"There is a real problem for the Labour Party if we don't look like we are taking these things seriously. But this is not just about the impact on the Labour Party, there is also the question here about what is the right thing to do.
The post referred to by Mr Cameron suggested the US has "plenty of land" to accommodate Israel as a 51st state, allowing Palestinians to "get their life and their land back".
It added that Israeli people would be welcome and safe in the US while the "transportation cost" would be less than three years' worth of Washington's support for Israeli defence spending.
Ms Shah added a note suggesting the plan might "save them some pocket money".