THERESA May has suffered a major blow with the loss of her Home Secretary Amber Rudd, who after days of pressure over the Windrush scandal and migrant deportation targets finally fell on her sword last night.

Despite colleagues rallying round Ms Rudd, the pressure from Labour and the SNP grew as more details emerged over the setting of targets on illegal immigration, which suggested she did not have a grip of her department.

Her opponents insisted she had misled Parliament.

The resignation not only means the Prime Minister has lost a major Cabinet figure and ally – Ms Rudd had been described as a “human shield” protecting the Prime Minister, who was her predecessor at the Home Office – but also pushes the balance between Brexiters and Remainers in the Cabinet in favour of the former.

READ MORE: Amber Rudd's resignation letter in full​

Speculation is already mounting that leading Brexiter Michael Gove, the pro-active Environment Secretary, who in the past has had major clashes with Mrs May, could be promoted to replace Ms Rudd.

For days the MP for Hastings had responded with concern and contrition over the individual stories experienced by members of the Windrush generation.

At one point, she blamed the culture in her own department for putting process over human beings.

Leaks to the press, perhaps from disgruntled civil servants, began to emerge.

Her position was already precarious on this issue but then more pressure was piled on her over that of target-setting and her apparent ignorance of the policy in her own department.

After appearing before MPs in the middle of last week to deny that her department had set targets for the deportation of illegal immigrants, she later had to accept targets were set.

READ MORE: Sajid Javid appointed as new Home Secretary after Amber Rudd resigns

Then, after a leaked memo at the end of last week showed she had been briefed on targets, her position became one of daily survival.

The pressure increased yesterday when The Guardian revealed details of a private letter from Ms Rudd to Number 10 setting out an “ambitious but deliverable” target for an increase in the enforced deportation of immigrants.

The letter, signed by the Home Secretary in January last year, stated that she was refocusing work within her department to achieve the “aim of increasing the number of enforced removals by more than 10 per cent over the next few years”.

Colleagues took to the airwaves to defend Ms Rudd, with Transport Minister Jo Johnson saying: “It’s unrealistic to expect a secretary of state to have seen every single memo that covers all of the policy areas.”

But Diane Abbott, the Shadow Home Secretary, said Ms Rudd had “done the right thing” and acted with “honour” in leaving the government.

She said: “The Windrush generation could not have had confidence in her. This was a massive Home Office scandal. In the circumstances, the right thing to do was to resign.”

READ MORE: Diane Abbott says Theresa May must face MPs following Amber Rudd's resignation

Joanna Cherry QC, the SNP’s justice spokesperson at Westminster, said Ms Rudd’s exit meant the focus would now turn to the Prime Minister’s time as Home Secretary.

She added: “Theresa May should be very worried now she’s lost her human shield.

"I’ve no doubt there will be more concerning revelations to come about the results of her toxic immigration policies and this time there will be no hiding behind Amber Rudd.”

READ MORE: Who are the contenders to be the new Home Secretary?

Ms Rudd, who was due to make another Commons statement to MPs this afternoon, telephoned Mrs May last night to tell her of her decision to quit.

Lib Dem leader Sir Vince Cable said Ms Rudd might have been able to mount a defence in the Commons, but not if there was more damning information to come out.

Speaking on BBC Radio 4’s Westminster Hour, he said: “She’s clearly jumped before she was pushed. I think there were some serious misjudgments and that’s why she’s had to go.”