Thousands of prison officers may join a national strike over pension reforms later this month despite it being illegal for them to do so, their union chief said on Thursday, a move that could paralyse jails that are crammed to bursting.

The action, if it came, would be an added headache for a government already facing the prospect of a strike on pensions by a coalition of more than 20 public service unions representing 2 million workers.

A full-blown strike by the Prison Officers Association (POA) would also put added strain on a creaking prison service trying to cope with record numbers of inmates and put the union on a collision course with the Ministry of Justice and the courts.

Steve Gillan, POA general secretary, said his 35,000 members were incensed over the government's stance on public-sector pensions and would break the law and support the mass walkout if necessary.

We are hoping that we can come to a settlement on pensions, but if we can't then we will support the November 30 day of action, he told Reuters.

That doesn't necessarily mean a strike, but we may well carry out other forms of action, he added, without specifying what they would be.

Asked if his members would be willing to break the law and join a national strike if negotiations achieved nothing, he replied: We are not ruling it out.

Prison officers are forbidden under the 1994 Criminal Justice and Public Order Act from taking industrial action, but Gillan said that hadn't stopped them in the past, most recently in 1997, when a row over pay led to a total walkout.

The Labour government at the time secured a High Court injunction against the union forcing it to call off the strike.

We have always been a union that says if the issue is right and if it is the will of our members then we would ignore the legislation, Gillan said.

He said full-blown industrial action involving an estimated 15,000 officers on duty across England and Wales would undoubtedly paralyse the prison service and cause an estimated 140 prisons to be in a state of lockdown until officers returned.

In reaction to the strike threat, the Ministry of Justice said it had well-established contingency plans for handling major incidents including stoppages.

We will be monitoring closely the impact that the strike action has on the prison estate. However, at this stage we do not anticipate that this will have a major impact on prisons, a spokeswoman said.

During the 1997 strike, prisoners were kept locked in their cells, while senior managers took charge of duties such as distributing meals, and visitors were turned away.

Gillan said any strike action would comply with health and safety laws and prioritise the welfare of prisoners.

We are not a renegade union ... and the reality is that we would have to look at minimum cover arrangements to make sure that prisoners are looked after, he said.

The prison population in England and Wales rose to an all-time high of 87,945 last week, close to its safe operational capacity of 89,255.

Upping the pressure on ministers, two more big unions, the GMB and Unite, announced this week that their members had voted in favour of a national stoppage. More are set to declare ballot results in the next two weeks.

(Editing by Steve Addison)