A British gay man lived a content life before he was contacted suddenly by the Child Support Agency (CSA), demanding he start paying £26 ($41, €32) a week for two children he technically fathered over a decade ago.

Mark Langridge, from Essex, helped a former lesbian couple who were desperate for children and ask him to donate his sperm so they could have children.

The 47-year-old had not seen the family he helped out of kindness create since 2004, he was not named on the birth certificates of the two children and played no role in their upbringing.

Now Langridge, self-employed and lives on a low income as a part-time bookkeeper and gardener was asked by the CSA to pay up. Payments will add up to £8,000 (€9960, $12840) before both children reach adulthood.

Langridge has been in a relationship with his partner Shaun Keeble, 37, for 17 years, and a few years ago have entered a civil partnership.

He told the Daily Mail newspaper: ‘Me and Shaun were not interested in having children of our own.

‘When the idea [of donating sperm] was suggested, we thought it sounded like a lovely thing to do. But the last few months have been a complete nightmare.

‘I feel as if I am the victim of a state-sponsored blackmail plot ... It was purely an act of kindness on my part and now I am being made to pay.’

Langridge and Keeble said they met the lesbian couple in a gay nightclub in Southend in 1997, and became good friends.

In 1998, one of the women was desperate to have children and then asked Langridge to help ,which he agreed out of kindness.

Langridge told the Guardian he only agreed to it after being given repeated assurances  the couple were financially secure and would need no financial input from him at any stage.

He was told by the mother he was not named on the child's birth certificate, and the mother's story would be she'd had a fling and the father had gone back to the US.

The woman asked her a year later to donate again because she wanted a sibling for her child from the same father.

He agreed and another girl was born in the spring of 2000, with the agreement he would not be named on the birth certificate.

For a few years Langridge remained in touch with the mother, and saw the girls at social events.

According to his accounts the women insisted on introducing him as their father.

The two couples eventually drifted apart, and apart from occasional letters from the girls Langridge has had no contact with the family since 2004.

The lesbian couple later separated, with the biological mother keeping both children and claiming benefits.

According to Langridge, the biological mother’s former partner lives near the former family home and sees the girls at weekends – but is not being chased for child support.

Instead, Langridge received his letter from the CSA in June.

He stressed he made sure the couple had the funds to bring up children before he donated his sperm.

The CSA said if Langridge had used an official sperm donation centre he would not have to pay child support.

However, in the 90s it was impossible for women to use official sperm banks unless they had a male partner, which is no longer the case today.

If Langridge would have made the donation today, he would not have been legally nor financially liable for the children.

Consequently, Langridge has called for the law to be applied retrospectively.

Langridge could contest the CSA’s ruling is by taking his case to a judicial review, which could cost in the region of £60,000, something he cannot afford.

The mother of the children told the Daily Mail in response: ‘It’s all being dealt with at the moment and I don’t want to comment.’

Langridge was quoted as saying in the Guardian: ‘Our only crime was agreeing to do someone a good turn.

'It is absurd that I'm being chased for the money while the children's other mother makes no contribution to their upkeep.

‘How can that be right in modern Britain in which we supposedly have equality?’