The Democratic Unionist Party's (DUP) 10 MPs will be abstaining on pretty much every Finance Bill vote on Monday night, I understand.
The Finance Bill sees MPs vote on measures announced in the Budget.
This is to put the Government on warning that the Party is close to ripping up the “confidence and supply” agreement that allows Theresa May to govern.
This is a serious blow to the Prime Minister’s authority, and it is all down to the DUP’s pessimism that Mrs May will amend in any significant way the EU Withdrawal Agreement that it sees as driving a regulatory wedge between Northern Ireland and Great Britain.
If Tory MPs see the DUP's industrial action as increasing the risk of an imminent general election, they will launch that long-planned and delayed coup against her pronto.