Lord Saatchi said it was impossible for anyone outside a marriage to know what is going on between a couple when he was asked about the breakdown of his brother's relationship with Nigella Lawson.

Asked about Charles Saatchi's divorce from the TV chef, the Tory peer declined to talk about the pair in person, offering only a vague answer about the complexities of marriage.

"There are many aspects of human life, let's consider them – persecution mania, love, sex, marriage, work. These are all areas which produce great difficulties in people, and family and marriage is certainly one of the most complex areas," he told BBC1's Andrew Marr Show.

Referring to his late wife, Josephine Hart, he added: "As Iris Murdoch said to Josephine Hart on many occasions, marriage is a private place. What Iris Murdoch was saying or suggesting was that it's impossible for anybody, even the best friends or relatives of the people involved, cannot know what really happens between a man and a woman.

"When somebody installs CCTV in people's bedrooms, which may be the next step of our surveillance state, then we'll have the answer but until then we don't know."

Saatchi's millionaire art collector brother and Lawson were granted a decree nisi – the first legal step to ending their 10-year marriage – at a hearing at the high court in London in July.

Lawson applied to divorce Charles Saatchi on the grounds of his continuing unreasonable behaviour.

News of their breakup emerged after pictures were published in June showing Saatchi holding Lawson by the throat during argument at a restaurant.

He dismissed the incident as nothing more than "a playful tiff" but then accepted a police caution for assault.

Saatchi later told the Mail on Sunday that the pictures gave a "wholly different and incorrect implication".

But he added: "I feel that I have clearly been a disappointment to Nigella during the last year or so, and I am disappointed that she was advised to make no public comment to explain that I abhor violence of any kind against women, and have never abused her physically in any way."

The couple have said they would not make any financial claims against each other.