Conservative chairman Brandon Lewis was “paired” with Liberal Democrat deputy leader and new mother Jo Swinson, meaning neither would take part in the Commons votes so her maternity leave did not tip the balance.
But the Tory MP then took part in two crunch Commons votes on Tuesday night to help Theresa May narrowly avoid damaging defeat on Brexit trade plans.
The Prime Minister admitted the Tories’ failure was “not good enough” and the party has insisted it was an error.
But Halifax MP Holly Lynch, who is expecting her first child in autumn, said the incident showed democracy is in a “very sorry state”.
“The practices around how Westminster functions are a long way from modern workplaces.
“This week’s debacle has demonstrated that during tight votes, when the Prime Minister’s reputation is on the line, the considerations for mother and fathers who are on parental leave, goes out the window. Not only would this be illegal in other workplaces, but it also lets the public down,” the Labour MP said.
“If MPs who are parents are denied a say in Westminster, democracy is in a very sorry state.”
Ms Lynch, who is 31, said the incident “risks putting people off getting involved in politics, if they think it is incompatible with becoming a parent”.
During Prime Minister’s Questions, Mrs May said: “The breaking of the pair was done in error. It wasn’t good enough and will not be repeated.”
The PM said the Chief Whip and the Tory party chairman had “apologised directly”.
Mr Lewis said it was an “honest mistake” made by the whips in “fast-moving circumstances”.
But Ms Swinson said it was “neither honest, nor a mistake” and the Government’s response was “not credible”.
Mr Lewis had missed earlier votes, but took part in the two crunch divisions on the Trade Bill, and Labour said it was “hard to credit” the account of the incident given by the party chairman and Chief Whip Julian Smith.