A foreign office statement, released on Friday, advises British nationals against "all but essential travel" to the kingdom. The ministry believes that attacks may be in the final stages of planning.
A previous warning had said simply that attacks against Westerners were "likely". Australia also said it had information that new attacks in Saudi Arabia might be close to being carried out.
The Australian Foreign Affairs and Trade Department said on Saturday that the level of its warning against non-essential travel to the country was unchanged.
The warnings followed an admission from Saudi Arabia that it had arrested nearly 600 people since suspected al-Qaida bombers struck Riyadh in May.
The conservative kingdom, birthplace of al-Qaida leader Usama bin Ladin, has faced intense pressure to crush al-Qaida cells since the September 11, 2001, hijacked-plane attacks in the US in which 15 of the 19 hijackers were Saudis.
It intensified its efforts against the network after the 12 May bombings in Riyadh which killed 35 people, including nine Americans and analysts say it has sharply stepped up cooperation with Washington.
In the latest crackdown, Saudi Arabia said on Monday it had arrested what it called Islamic militants and seized large amounts of weapons and explosives, including bomb-belts.
Saudi Arabia has also tightened up monitoring of charities that have been accused of channelling funds to militant groups worldwide, shutting down some overseas branches of domestic charity groups.