Civil servants at HMRC are “definitely up for the fight”, a union said as they walked out in a dispute over pay and conditions.

PCS members at HMRC in East Kilbride, South Lanarkshire, and in Newcastle began targeted strike action on Wednesday which will see them down tools for 18 days between now and June 2.

They claim they have not had a proper pay rise from the Government for 13 years and are calling for a 10% increase, saying they “will not accept being treated like the poor relation”.

Union members manned picket lines outside the HMRC offices in East Kilbride and Benton Park View in Newcastle on Wednesday morning.

The union said the strike will hit the employer helpline and the construction industry scheme helpline and webchat services, and it warned of a severe impact on the student loans unit.

John Davidson, PCS branch president for HMRC in East Kilbride, said members are taking action to get a fair pay rise from the Government.

He told the PA news agency: “Our members have not had a proper pay rise from the Government for 13 years in the middle of the worst cost-of-living crisis that any of us have ever suffered.

“When the cost of living is going up exponentially, the cost of food, fuel, heating bills etc is increasing again and again, it’s unfair for our members to have to survive on no more than a 2% pay rise, which is the last official offer that we’ve had, so that’s why we’re out on strike today.

“Ten per cent was the initial pay claim but obviously the cost of living continues to increase and our members aren’t just suffering that now, they have suffered that for over a decade, so the actual cost of their wages compared to what they were 13 years ago is significantly lower when you look at the rising cost of living, so 10% is our baseline – but the offer from the Government is nowhere near that at the moment.

“Since we served notice of this action on employers, we’ve recruited hundreds of additional members between here and the other office in Newcastle that’s out on strike today so they’re definitely up for the fight.”

He said 19,000 of the union’s members across HMRC are on the national minimum wage and claimed the low pay is having an impact on staff retention, and therefore a knock-on effect on the ability to deliver services.

Mr Davidson said: “This is not just about selfish civil servants taking action in defence of pay, this is about us defending public services.”

PCS general secretary Mark Serwotka said: “In HMRC alone, almost one in three staff are now on the national minimum wage. In DWP (the Department for Work and Pensions), one in five staff are having to claim in-work benefits. These people are the Government’s own workforce, who are suffering the consequence of year after consecutive year of meagre pay rises leaving tens of thousands of them in financial crisis.

“We will not accept being left behind. We will not accept being treated like the poor relation. PCS members are determined to keep the pressure on until the Government improves its offer to us.”

A UK Government spokesperson said: “Our pay remit guidance recognises the hard work and vital importance of civil servants by offering the highest pay increase in 20 years, in line with forecast wage growth across the economy.

“The deal is also fair to the taxpayer and supports the Government’s promise to halve inflation this year, which will help everyone’s incomes go further.

“Industrial action should always be a last resort and dialogue with unions will continue. We urge them to recognise what is reasonable and affordable, as the whole country faces these cost-of-living challenges.”