Portas, who last month surprised her partner Melanie Rickey as they became one of the first couples in the UK to convert their civil partnership to a same-sex marriage, told how her brother Lawrence Newton helped her and the fashion journalist conceive the boy, named Horatio.
In an interview with the Times Magazine she said her brother had said he would be “honoured” when she asked him to donate sperm for the IVF treatment and he was present as Ms Rickey gave birth two years ago.
She said: “Lawrence was the first one there (with us) and he picked him up and it was just very emotional. I said to him ‘thank you’ and he said ‘it’s my pleasure’, and we just held each other.
“And now when I look at Horatio, he is a complete mix of (Ms Rickey) and me. Of course now I know that it could only ever have been Lawrence who was Horatio’s father.”
The couple, who have three children together, had a civil partnership conversion at Westminster City Hall in London at 12.01am on December 10, one minute after the law came into effect.
Westminster superintendent registrar Alison Cathcart presided over the conversion of Ms Portas and Ms Rickey’s civil partnership, and said she saw it as the “final step in equality for gay marriage” and Westminster City Hall was “very proud to play our part in it”.
At the time, Ms Portas described the occasion as “absolutely magical” and said she had been planning it for about a month.
Portas is one of country’s foremost authorities on retail and spoke at the Yorkshire International Business Convention with the Dalai Lama in June 2012.
In 2011, she was appointed by the Prime Minister David Cameron and Deputy Prime Minister Nick Clegg to lead an independent review into the future of the high street, with Rotherham one of the first towns to be targeted under her action plan.