In what is the latest attack since Jeremy Corbyn was overwhelmingly voted by party members to lead Britain’s Labour Party in September, some Labour parliamentarians are attacking his stance against proposed airstrikes in Syria, going as far as threatening to resign.

"There will be resignations among senior members of the shadow cabinet over this," an unnamed senior shadow cabinet member told the BBC.

Corbyn sent a letter to all Labour parliamentarians hours after Prime Minister David Cameron proposed a plan for increased military action against Syria in Parliament Thursday, saying he was not convinced by the prime minister’s case for airstrikes.

“I do not believe the prime minister’s current proposal for airstrikes in Syria will protect our security and therefore cannot support it,” Corbyn wrote.

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Several shadow ministers from the Labour Party have said that they would resign their positions if Corbyn demanded they vote uniformly instead of having a free vote over allowing United Kingdom airstrikes in Syria when the issue is presented in parliament.

So @fionamacmp + @spellar MP call on Jeremy Corbyn to resign 4 upholding Labour Party policy on #Syria. Using war to push their real agenda. — Lee Brown (@leejamesbrown) November 27, 201

According to media reports, half of the shadow cabinet supports the airstrikes in Syria and agree with Cameron’s arguments that the U.K.’s security is under threat by the Islamic State group.

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It appears some senior members of the Labour Party were offended by the letter, as they argue the letter should not have been sent before an agreement was reached among Labour’s “shadow cabinet” — those chosen by the party leader to hold their counterparts in the ruling party to account.

But not all member of the shadow government saw it that way. Shadow International Development Secretary Diane Abbott insisted that Corbyn’s views were those of the party’s grassroots, saying his mandate to lead was too large for his cabinet to vote against him. "You cannot have a shadow cabinet voting down the leader of the Labour Party who has just been elected with the biggest mandate in history," she said.

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Shadow Employment Secretary Emily Thornberry agreed with Corbyn that the prime minister failed to make the case for airstrikes in Syria, adding there was a "brutally honest debate going on" within the Labour Party. However, she argued that members cannot be forced — or “whipped” — to vote in a certain way on such an important issue as war.

"When it comes to an issue of war it is something that people think very profoundly about. We do usually act collectively, but I think on issues like this there are times when people cannot stick to a whip which is imposed," she said.

In 2013, U.K. lawmakers voted against military action against Syrian President Bashar Assad, but they did later approve British airstrikes against the Islamic State group in Iraq.

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