The police and MI5 are investigating whether members of the two terrorist cells responsible for the bombing attacks on London met up at an adventure centre in north Wales last month.

Two of the bombers on the run after the failed 21 July attacks are believed to have been at a white-water rafting centre in Snowdonia on 4 June at the same time as two of the 7 July suicide bombers, Shahzad Tanweer and Mohammad Sidique Khan.

The growing evidence of direct links between the two units has provoked fears that there could be other bombing cells at large.

The disclosure comes as a fifth bomb was discovered dumped in bushes in London, which contained homemade explosives similar to those used in the previous eight devices.

Three men have been arrested in connection with the attempted attacks in London last Thursday, but despite reports that one of the bombers had been caught, it appears that all four are still on the run.

A massive manhunt is continuing in which the police and security services are working around the clock to locate the failed bombers, amid fears that some will strike again.

Evidence has started to emerge that suggests the cell had connections with east Africa, in particular Ethiopia and Somalia.

Police are believed to have identified all four of the would-be bombers, whose explosives failed to detonate on three Tube trains and a bus.

The names of two of the terrorists are understood to be among the names of a group of people who went to the National White Water Centre, at Canolfan Tryweryn, near Bala, on 4 June. On the same day, Khan, 30, and Tanweer, 22, both from Leeds, were also at the centre.

Photographs of the pair enjoying white-water rafting were later published by the media. Detectives are investigating whether the named suspects are the same people who carried out the failed attack in London on 21 July.

If all four terrorist were at the adventure centre at the same time, the police will want to discover whether they were meeting to discuss the forthcoming terrorist campaign, or linking up with a bomb-maker or al-Qa'ida planner.

The discovery that the units both used a similar-looking type of chemical explosive, both targeted three Tube trains and a bus, and were prepared to carry out suicide attacks in London, points towards some form of co-ordinated operation.

Anti-terrorism sources said yesterday that they were keeping an open mind, but admitted that the involvement of a wider network looked increasingly likely.

The remote rafting centre in north Wales, which attracts 80,000 visitors a year, would have made an ideal meeting place for the bombers, being far away from London and difficult for surveillance teams to go unnoticed.