The ballot has been triggered after at least 54 Tory MPs outlined their dissatisfaction in letters to the chairman of the party’s 1922 Committee.
Ahead of the evening poll – which is due to take place between 6pm-8pm and which could lead to a leadership contest – Mrs Mortimer has backed the Prime Minister and added that “we would not be where we are today without Boris”.
Writing on her Facebook page, she said on Monday: “Many of you will have seen that a vote has been called tonight on whether Boris Johnson should remain leader of the Conservative Party.
“I am currently slipped and away from Parliament, but I will be voting by proxy – and let me be clear – I will be voting for Boris to stay.
“As I have consistently stated, mistakes were made in Downing Street during lockdown.
"My households spent lockdown shielding – my son is a carer for disabled adults and so we followed the rules to protect those vulnerable members of society, and I know many of you made similar sacrifices.
“Quite rightly the Prime Minister has apologised and paid the fine issued to him for the party he did attend.
"However, I have spoken to many people in Hartlepool who, like me, feel that this matter should now be closed.
“We would not be where we are today without Boris.
“Hartlepool voted overwhelmingly for Brexit and Boris got it done.
“Faced with the unprecedented circumstances of the last few years Boris has consistently got the big calls right – fronting the fastest and world leading vaccine rollout in response to the Covid-19 pandemic and leading the West’s response to Putin’s invasion of Ukraine.
“Boris cares about Hartlepool and is the only Prime Minister who has committed to supporting increased investment across the Tees Valley.
“You voted for me, and you voted for much-needed change in Hartlepool.
"Whether it be nuclear, education, transport or our town centre – this Government is serious about ensuring Hartlepool can prosper and it is my belief that Boris is the driving force behind this.
“He delivers on the priorities of Hartlepool and the Tees Valley, and in my view, remains the best leader for the country – so I will continue to back Boris.”