Prison Officers across England and Wales are defying a High Court injunction to continue strike action over pay.

Earlier, the justice secretary Jack Straw warned the striking prisoner officers that the unofficial action was illegal and “wholly unjustified”.

Prison officers walked out at 7am this morning over low pay and thousands are refusing to return until 7am on Friday morning.

However, after the Ministry of Justice successfully secure a High Court injunction against the strikers, officers in Bristol and Canterbury returned to their posts.

Thousands of prison officers threatened to defy the order, however.

Addressing workers outside of Liverpool prison, shop steward Steve Baines said: “I’ve spoken to Steve Gough, the national vice-chair [of the Prison Officers’ Association], and he expressed the view to us: ‘Tell them to shove it up their arse, we’re sitting it out’.”

Addressing striking officers outside Wormwood Scrubs prison in west London this morning, the Prison Officers’ Association’s (POA) John Hancock said a “national strike” was underway, with “excellent” support at Wandsworth, Pentonville and Reading.

Mr Straw condemned the strike as “deeply regrettable and wholly unjustifiable”.

The justice secretary said: “Our first concern in this situation is to protect the public.

“We have in place tried and tested contingency measures to ensure the security of all prisons across England and Wales is maintained. We will also ensure that prisoners receive meals and emergency medical attention.”

The POA warned earlier this month that a strike was possible after a ballot of members came out in support of industrial action.

Nearly nine in ten of those balloted voted for industrial action “up to and including a strike”.

The prison officers are aggrieved at the government’s latest pay offer, which they warn amounts to a pay cut.

A pay review body recommended an annual pay rise of 2.5 per cent. As with nurses, the government said this should be staged, beginning with a below-inflation 1.9 per cent pay rise.

The POA warned this came when morale among prison staff was already “rock bottom”.

Colin Moses, chairman of the POA, told the BBC the decision to strike had been taken only after “after two years of frustration and two years of below-inflation pay awards”.

He continued: “These are professional men and women who every day look after those committed by the courts to our prisons.

“Surely we should be treated as well, if not better, than the prisoners we look after.”

The Police Federation of England and Wales said the strike was a result of the government’s attitude to public sector pay and said it empathised “wholeheartedly” with the POA.

Alan Gordon, vice chairman of the federation, said: “This is the result of the government’s wish to force through settlements rather than wishing to negotiate solutions.

“However, the irony will not be lost on those officers when, yet again they will be expected to deal with the problems that will arise in the prisons as a result of this industrial action at a time when those very officers themselves are denied a fair pay rise due to government intervention, but are prevented by criminal law to take similar industrial action.”