Mexico's outgoing President Andres Manuel Lopez Obrador in 2019 sent a letter to the king asking that he "publicly and officially" acknowledge the "damage" caused by the 1519-1521 conquest, which resulted in the death of a large part of the country's pre-Hispanic population.

"Unfortunately, this letter was never replied to directly, as should have been the best practice in bilateral relations," Sheinbaum said in a statement.

Mexico had in July invited Spain's Socialist Prime Minister Pedro Sanchez to the swearing-in ceremony on October 1, the statement added.

Spain announced late on Tuesday that it would boycott the ceremony because the king was not invited, calling his exclusion "unacceptable".

"For this reason, the Spanish government has decided not to participate in the inauguration at any level," the Spanish foreign ministry said in a statement.

Mexico published the guest list a week ago for the inauguration of Sheinbaum, who will be the country's first woman president following her left-wing ruling party's landslide June election victory.

King Felipe VI was not on the list, which includes regional leftist leaders as well as US First Lady Jill Biden.

"The head of state, the king of Spain, always attends all swearing-in ceremonies and therefore we cannot accept that in this case he should be excluded," Spanish Defence Minister Margarita Robles told journalists in Madrid on Wednesday.

"And therefore if the head of state is excluded, Spain will not be represented, which we are very sorry about because the Mexican people are a brotherly people."

While Mexico and Spain have close historical and economic links, relations between the Latin American nation and its former colonial ruler have been strained since Lopez Obrador -- an ally of Sheinbaum -- took office in 2018.

He has frequently complained about Spanish companies operating in Mexico and twice declared during his mandate that his country's relations with Spain were "on pause".

Madrid has rejected his demand for an apology for the events of the Spanish conquest five centuries ago.