Prince William has said Prince Harry has yet to ask him to be his best man at his wedding to Meghan Markle.
William joked that the matter could be "a sensitive issue" as he met volunteers at the Campaign Against Living Miserably (Calm), which works to change culture around masculinity and help men who are at risk of suicide.
Harry (33) and his American actress fiancee (36) are due to marry in May at Windsor Castle.
Discussing the charity's Best Man video project, which encourages men to open up to their friends, William joked about his younger brother's upcoming wedding to Ms Markle in May, quipping: "He hasn't asked me yet - it could be a sensitive issue."
The second-in-line to the throne also said he is "still working" on the clash between the May 19 wedding at St George's Chapel, and the FA Cup final the same day.
William also spoke more seriously about how losing his mother, Diana, Princess of Wales, at a young age helped create a special "bond" with Harry.
Speaking alongside former footballers Rio Ferdinand and Jamie Moralee, who have also campaigned on mental health issues in the past, William said: "My brother and I's relationship is closer than it's been because of the situations we've been through.
"Losing our mother at a young age has helped us to travel through that difficult patch together."
Calm is a partner of Heads Together, the mental health campaign led by William, alongside the Duchess of Cambridge and Harry.
William also spoke to a group of young mental health volunteers, telling them that older British men are still largely unable to discuss mental illness due to their "stiff upper lip".
He said: "I still believe the war had a lot do with it - particularly the men.
"People just did not know how to deal with it."
Volunteer Alex Stanley (32) told William that he became interested in men's mental health from the age of 14, when his brother killed himself.
Mr Stanley, who ran the London Marathon last year for the charity, later said it was "absolutely fantastic" to have William coming to discuss the issue.
"The number of people a couple of years ago who knew what the male suicide rates were in the UK - that 76 per cent of all suicides are men - not many people knew that," he said.
"Thanks to the duke and Heads Together, the issue of male suicide has been in the limelight. That can only do good things to improve mental health and prevent suicide."
Fellow volunteer Phil Hill (25) added: "William is going to get through more to older generations and people who perhaps need a bit more convincing in terms of talking about feelings."
Male suicide is the single biggest killer of men under the age of 45 in the UK, according to Calm.
Ferdinand spoke earlier this year about the pain of losing his wife, Rebecca.
He appeared in a video released by Heads Together, in which he spoke to Moralee about the importance of opening up about emotions.
Earlier on Wednesday, William donned surgical scrubs to learn about the pioneering robotic surgery of a leading cancer hospital - then joked that its surgeons were computer game fanatics.
Dressed in a blue top, trousers and cap, and wearing a pair of slip-on foam rubber Crocs, the future king was taken into the operating theatres of the Royal Marsden Hospital to watch tumours being removed from patients with the help of the machines.
The Da Vinci robot allows surgeons to remotely cut away the cancerous growths as they operate two handheld controls connected to a 3D monitor located a few feet away from the patient.
Dexterous consultants have been taught the skills needed to handle the robot, which allows them to operate without resorting to major invasive procedures, and William watched with fascination as Anne White (67) from Newton Abbot, Devon, had a tumour cut away from her tongue.
He peered at the 3D monitor as lead surgeon Professor Vin Paleri talked him through the procedure and then later watched another operation to remove a tumour from the base of 63-year-old Charles Ludlow's oesophagus.
William, who is president of the Royal Marsden NHS Foundation Trust, talked about the skills of the surgeon when he met cancer patient Joe Omar (63) and his wife Lynn (59) and their daughter Leila (27).
The duke joked with the 63-year-old, comparing the Da Vinci robot to a computer game, saying: "It's the same as the PlayStation Gear.
"You can see all the doctors have done PlayStation. They let me watch but not to have a go."
Meghan Markle's half sister Samantha Grant has insisted Thomas Markle Snr. will definitely be at the couple's nuptials at St George's Chapel, Windsor Castle, on May 19 unless a "plane crashes through his roof".