They are Jacky Lai Ming-chak from Sai Kung, Ho Chi-wang from Central and Western district, Lee Ka-wai of Tuen Mun, To Ka-lun of Yuen Long and Christine Wong Yi of Eastern district.

Lai, 29, a former member of the Neo Democrats, apparently left for Switzerland at the end of February before heading to Britain with his family.

"Lai left Hong Kong with the help of a good friend who was active in Tseung Kwan O community work and is a close friend of wanted activist Ray Wong Toi-yeung," said Eastweek, quoting familiar sources.

"He had been asking around about ways to immigrate and started to panic after Gary Fan Kwok-wai was arrested for subversion for taking part in the pro-democracy camp's primary elections last July, as he was second on Fan's list," sources added.

Ho, 28, arrived in Britain at the beginning of the year. Asked why he had not been in office for a while, staff said he was sick. Ho posted on social media last month that he was suffering from depression.

Sources said Ho was worried after the arrest of Michael Pang Cheuk-kei - who is also being remanded along with Fan for subversion - as Ho was second on Pang's list in the primary elections.

Another district councillor, Lee, is believed to have arrived in Britain in mid-March. He announced on social media on March 24 that he would not pledge allegiance to the Basic Law and HKSAR, though he did not say whether he would resign.

To, 62, and Wong, 38, left the SAR in March. To said he tendered his resignation last Wednesday, while Wong resigned due to "health problems."

A non-pro-establishment politician said dozens of district councillors are considering emigration and some will decide after reading the details of the oath.

Pro-establishment sources believe pro-democracy councillors are worried that they will have to return their salaries after being disqualified or be prosecuted for breaching the national security law, so they are leaving as soon as possible.

Meanwhile, the Constitutional and Mainland Affairs Bureau said the government is busy "perfecting Hong Kong's electoral system and arranging three elections in the next year" and so has no plans to hold by-elections for the district councils yet.

Meanwhile, former lawmaker Dennis Kwok Wing-hang announced on social media last night that he has decided to part ways with the Civic Party.

Kwok, who was disqualified last November, thanked the party and apologized for his resignation.

It was earlier reported that Kwok left Hong Kong last November with his family. Canadian media reported earlier this month that he is in Canada, hoping to start a new life in the country he was born.