Britain's biggest mosque is to fully reopen on Saturday following a costly rebuild after the complex was partly destroyed by a huge fire in 2015.

The Ahmadiyya community's five-storey Baitul Futuh Mosque, in the Morden district of south London, can accommodate up to 13,000 worshippers.

The huge prayer hall survived the blaze but the mosque's administrative block housing function rooms was gutted.

It has been rebuilt using light-coloured stone imported from Portugal at a cost of £20 million ($24 million), paid for entirely by members of the community, according to mosque officials.

Mohammed Nasser Khan, vice president of the Ahmadiyya Muslim Association UK, said the new complex would offer improved facilities including for children's education.

"But more importantly for me personally, and I think for many members of the community, is that our neighbours and all the people that visit these premises, that they have a facility that they can be proud of also," he told AFP.

About 1,500 people are expected to attend the re-opening, including British politicians, diplomats and religious leaders.

The Ahmadiyya sect has about 35,000 followers in Britain, out of a total of about four million Muslims.

They are most numerous in Pakistan, where they are considered as heretics and subject to persecution.

A sixteen-year-old boy was accused of intentionally starting the London fire, but cleared of arson at a trial in 2017.

Resolutely pacifist, Ahmadis are notably opposed to the military interpretation of jihad (holy war). "Love for all, hatred for none" is written at the entrance of the Morden complex.