...."Boris Johnson has "already driven a bulldozer through the constitution", said Plaid's Westminster Leader, Liz Saville Roberts.
The law, which should gain royal assent on Monday, aims to stop the UK exiting the EU with no deal on 31 October.
Number 10 has been contacted for comment.
No prime minister has ever successfully been impeached but Mr Johnson himself previously supported a bid to impeach Tony Blair when he was PM in 2004.
It is method by which Parliament can try individuals for high treason or other misdemeanours, but it is now considered to be obsolete.
The Brexit cross-party bill - which requires the prime minister to extend the exit deadline until January unless Parliament agrees a deal with the EU by 19 October - was passed on Friday.
sut the PM has said he would "rather be dead in a ditch" than ask for a delay.
Foreign Secretary Dominic Raab said previously that the government would abide by the law but would "look very carefully" at its "interpretation" of the legislation.
But legal experts have warned the prime minister could go to prison if he refuses to comply with the new law.
Ms Saville Roberts, MP for Dwyfor Meirionnydd, said she would be telling opposition party leaders that they should be ready to impeach Mr Johnson if he ignores the new law.
"Boris Johnson has already driven an bulldozer through the constitution, so no longer are ideas like impeachment farfetched," she said.
"If the prime minister becomes a law-breaker, we have an even stronger case for impeachment than the very cause he advocated back in 2004.
"No one is above the law, Boris Johnson shouldn't risk finding that out the hard way.
"No PM would want to play fast and loose with views he held on such matters in the past."