At least 45 others were injured in the explosion between two underground stations in the destination yesterday (April 3) afternoon.
The FCO updated its travel advice to Russia shortly after the blast, which forced the closure of all metro stations in St Petersburg.
The FCO said: “There are reports of explosions on the St Petersburg metro resulting in a number of casualties. If you’re in the area you should take extra care and follow the advice of local authorities.”
Russia and Eastern Europe specialist Regent Holidays had a number of clients arriving in the city yesterday.
“They have all been reported as safe and are accommodated at one of our most popular hotels, which is a short distance from the scene of the incident,” it said.
“Over the coming days we will maintain close contact with our clients, via our agents, and very much hope that they will all be able to continue their visits to this wonderful city as originally intended."
Regent’s agents in the city confirmed that the blast happened between Sennaya Ploshchad and Tekhnologichesky Insitut, two key stations near the centre and many of its attractions.
It added: “Our local agents have not reported any further significant disruption, although it is likely that there will be extra security around the major sites. We are also closely monitoring the advice from the Foreign Office, as the safety and security of our clients is paramount.”
Andrew Glenister, product manager of Real Russia, added that there was “currently no reason to believe that this tragedy will be repeated”.
A number of lines feature the Russian city as part of their Baltic Sea sailings during the summer.
The FCO travel advice for Russia warns of a “high threat from terrorism” and advises visitors to remain vigilant in all public places.
Authorities in St Petersburg have declared three days of mourning following the attack.
Russian President Vladimir Putin, who was in St Petersburg, which is his home city, when the bombing occurred, visited the scene on yesterday evening to lay flowers at a makeshift shrine.
No group has so far claimed responsibility for the attack, BBC News reports.
A native of Kyrgyzstan, who had obtained Russian citizenship, has been identified by Kyrgyz security services as the man suspected of the train bombing.
There are conflicting reports as to whether he was a suicide bomber.
Kyrgyz state media said the country’s security authorities were "maintaining contact with the Russian secret service for further investigation".
