King Charles Gives a Smile as He's Spotted in London amid Cancer Treatment with Queen Camilla

Queen Camilla recently returned to royal work following a break surrounding the Easter holiday and her anniversary with the King

King Charles and Queen Camilla are back in London.

On April 17, the royal couple was photographed in a car leaving Clarence House, their royal residence in London. The Queen, 76, rode in the passenger seat while the King, 75, sat in the back on the same side, both giving a smile and wave at onlookers.

The spotting came just one day after Queen Camilla resumed her royal duties after a few weeks spent largely out of the public eye during a break for the Easter holiday as well as her 19th wedding anniversary with the King on April 9.

Meanwhile, King Charles continues to work mainly behind the scenes since he announced his cancer diagnosis in February. The announcement came weeks after he underwent a procedure to treat an enlarged prostate in January, during which “a separate issue of concern was noted,” the palace said in a statement. (They clarified that the monarch did not have prostate cancer.) King Charles began "a schedule of regular treatments" and postponed most public-facing duties, following medical advice.

Related: King Charles Addresses Cancer Concerns During Surprise Easter Walkabout: 'I'm Doing My Best'

At the end of March, King Charles made his first major public appearance since sharing the cancer diagnosis as he and Queen Camilla attended a church service at St. George's Chapel at Windsor Castle on Easter along with other members of the royal family.

The pair spent time in Birkhall, a favorite Scottish hideaway, to mark their wedding anniversary earlier this month. The King and Queen often retreat to the residence, close to the royal family's Balmoral Castle and estate, around this time every year.

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While the Queen continues her royal duties, King Charles is reportedly "frustrated" by his current limitations.

Peter Phillips, the King's nephew, told Sky News Australia's The Royal Report in March, "I think, ultimately, he's hugely frustrated. He's frustrated that he can't get on and do everything that he wants to be able to do."

“But he is very pragmatic, [and] he understands that there's a period of time that he really needs to focus on himself," he continued.

Just before Easter, a royal source told PEOPLE that King Charles is "progressing well" and there is an "amplified confidence" about the sophisticated treatment he's receiving.

"He is positive, the doctors are optimistic," the source said.

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