John Banks has lost a paternity case.

A High Court judge has ruled the former Cabinet Minister and Auckland mayor is the paternal father of Antony Shaw.

Antony Shaw filed legal proceeding against John Banks, alleging Mr Banks had a relationship with his mother in the 1960s - and that he is Mr Banks' biological son.

Shaw asked the judge to declare that Banks is his next of kin - an order which would have legal implications for Banks' will.

In a just released decision, Justice Patricia Courtney ruled that on the balance of probabilities, and taking into account Banks’ refusal to undergo a DNA test or engage in the legal process, she was prepared to make a legal declaration regarding Shaw’s paternity claim.

During the hearing, the court heard John Banks urged the mother of his illegitimate child to have an abortion and then supplied drugs to make her miscarry and pressured her to take the necessary dose.

The woman, who said in a sworn affidavit she became pregnant after having sex with Banks in a Hamilton motel, claims she refused to take the pills and the two-term Auckland mayor ended their relationship.

She also claims he encouraged her to form a new relationship then pretend her next sexual partner was the child's real father. She subsequently did - perpetrating the lie for three decades, she says in a sworn affidavit.

In a statement, Shaw’s lawyer, Jacque Lethbridge, said her client was "disappointed that he had to endure a stressful, costly and public court action that could have been avoided had Banks engaged in private discussions which Shaw and his family attempted on several occasions, and had he agreed to a DNA test."  

"Mr Shaw pursued this action not only to obtain certainty as to his parentage but also to remove the stigma of illegitimacy that he has had to deal with since learning of the true situation in 1999. 

"Mr Shaw has also pursued this litigation for all those people that may, through no fault of their own other than by virtue of the fact of their birth, have found themselves in a similar situation to him," she said. 

Judge Courtney said she accepted there was a “high degree of similarity” between photographs of Banks and Shaw supplied to the court in evidence and that Shaw’s mother Pamela Mayes had been in a sexual relationship with Banks in 1969 - the year Shaw was conceived.