The Duke and Duchess of Sussex moved from Kensington Palace in April before the birth of their son Archie (Picture: PA).
Major renovations at the Duke and Duchess of Sussex's home near Windsor have cost the taxpayer £2.4 million.
Graham Smith, from the Republic campaign group, compared the money spent on Frogmore Cottage with a charity's funding of a centre for military veterans with post-traumatic stress disorder (PTSD).
In a tweet linking to an article on the construction of a support hub for Royal Marines in Lympstone, Devon, he said: "A charity spent £2.4 million on a support centre for marines suffering PTSD.
"The taxpayers then spent the same amount on a luxury private home for Harry and Meghan."
Speaking to The Sun, Mr Smith noted that the renovation bill was part of an increase in overall expenditure by the Royals: "This year's increases are outrageous at a time of widespread spending cuts."
The annual royal accounts were released on Monday.
Sir Michael Stevens, Keeper of the Privy Purse, who is responsible for the monarchy's accounts, said: "The property had not been the subject of work for some years and had already been earmarked for renovation in line with our responsibility to maintain the condition of the occupied royal palaces estate.
"The building was returned to a single residence and outdated infrastructure was replaced to guarantee the long-term future of the property.
"Substantially all fixtures and fittings were paid for by their Royal Highnesses."
Harry has close links with the Royal Marines, having been appointed as Captain General Royal Marines in December 2017, succeeding his grandfather, the Duke of Edinburgh, in the role.
He has also done extensive work with his fellow veterans. In 2014, he founded the Invictus Games, a multi-sport event for wounded or ill armed services personnel.