Prime Minister Rishi Sunak and Labour leader Sir Keir Starmer are set to attend the Japanese state banquet at Buckingham Palace, just over a week before the General Election.

Mr Sunak and Sir Keir have been invited to the grand affair hosted by the King in honour of Emperor Naruhito and Empress Masako next Tuesday evening and are currently both expected to be there.

A Japanese Foreign Ministry official said the emperor and empress’s visit would not be a political one and it was hoped it would forge “friendly relations across generations” between Japan’s imperial family and the British royal family.

The King, who is still undergoing treatment for cancer, and the Queen will fete the emperor and empress with a traditional welcome at Horse Guards Parade, a luncheon at the Palace, a picture gallery exhibition and then the lavish banquet.

Heir to the throne the Prince of Wales will greet the royal guests at their hotel on behalf of the King on Tuesday morning.

The Princess of Wales, who returned briefly to the spotlight at Trooping the Colour last weekend despite her ongoing cancer treatment, is not expected to take part in any element of the visit.

The Japanese Foreign Ministry official said: “In Japan, Their Majesties (the emperor and empress) are not engaged in the political or the economic spheres so we don’t have any plans to promote our economic relationship by this visit itself.

“This visit is actually to promote our friendly bilateral relations in culture, science.”

The official added: “The deep and long ties between the imperial family and the royal family are also a cornerstone of the Japan-UK relationship.

“So it is expected that this visit will promote friendly relations across generations between the imperial family and the royal family.”

There was no occasion during Elizabeth II’s 70-year reign of an incoming state visit being staged in the timespan between the calling of a general election and polling day.

The state visit has been slightly adapted because of the July 4 poll, with missing elements including the usual Downing Street talks with the Prime Minister, a speech to the Palace of Westminster by the visiting head of state, and meetings with party leaders.

The banquet takes place the day before Mr Sunak and Sir Keir face one another in a head-to-head TV debate on June 26 – the last of the election campaign.

The dinner, in the ballroom of the Palace, will see Mr Sunak and Sir Keir dress in white tie, process in and take their seats around the horseshoe-shaped table to hear speeches and toasts by Charles and the emperor.

A prime minister usually sits near the head of the table, with the leader of the opposition positioned further down.

Mr Sunak faced fierce criticism for returning early from the D-Day 80th anniversary commemorations and missing the main international ceremony to record a TV interview.

Emperor Naruhito, ruler of the Chrysanthemum throne, and his wife are arriving on Saturday for a seven-day stay and carrying out private engagements before the official state visit begins on June 25.

The emperor will take a special trip on Monday to see the Thames Barrier – one of the largest movable flood barriers in the world – reflecting one of his great passions.

As a student at Oxford University for two years in the 1980s, his research focused on water transport on the River Thames in the 18th century.

In his 1993 memoir The Thames And I, he described the period at Oxford as the happiest time of his life.

Empress Masako, a Harvard and Oxford-educated former diplomat, struggled for many years after developing “adjustment disorder”.

She withdrew from official duties for more than a decade.

Asked whether considerations of the empress’s health had been taken into account, the Japanese Foreign Ministry official said: “This is a very sensitive question, so I normally cannot make any comments on health but she is very happy to visit the United Kingdom, and the programme has not too much depth for Her Majesty.”

Empress Masako is expected to attend all engagements involving the King and Queen, as well as visiting the Young V&A museum and taking a trip to Oxford, where she studied at Balliol College.