By Duncan Gardham, Security Correspondent

The four men – three aged 19 and one aged 24 – are accused of raising funds for terrorism and travelling to Pakistan for terrorist training.

The men were arrested in connection with a “major counter-terrorism investigation” known as Operation Pitsford.

The men were detained by officers from the West Midlands Counter Terrorism Unit at their home addresses in the Sparkhill area of the city early on Tuesday morning.

A spokesman for West Midlands Police said the arrests were “pre-planned” and armed officers were not involved.

All four are being held under the Terrorism Act 2000 and detectives have an initial 48 hours to question the suspects before charging, releasing or applying for a warrant of further detention.

Eight people arrested over the last two months have already been charged. Three are alleged to have been plotting a suicide bombing campaign of “mass murder” on the streets of Britain.

Two of those are said to have travelled to Pakistan where they received terrorist training and made alleged suicide videos before returning to build a bomb.

The men were under surveillance as part of an “intelligence led operation” by MI5 and the police as they discussed their alleged plans, which were picked up by listening devices.

Three other men allegedly helped raise £12,000 by collecting for charities which was said to be used for the purposes of planning their attacks. A seventh man and a woman have been accused of failing to inform police.

Those previously charged include Rahin Ahmed, 25, from Moseley, Birmingham, who allegedly helped others to travel to Pakistan for training in bomb making, using weapons and mixing poisons.

He is also said to have collected money and invested and managed the money which was to be used for “terrorist acts” according to charges.

Irfan Nasser, 30, from Sparkhill, Birmingham and Irfan Khalid, 26, of Sparkbrook, Birmingham, are accused of plotting their attacks between Christmas Day last year and their arrest in September.

The charges say that they travelled to Pakistan for training in terrorism including bomb making, weapons and poison making.

While there they are said to have made a “martyrdom film” and later began planning a “suicide bombing campaign or event.”

On their return they allegedly provided information to others about training in Pakistan and were “advising and counselling” about explosives and detonators.

The men went on to purchase “components and chemicals for a home made explosive device,” the charges state and constructed a “home made explosive device for terrorist acts.”

They tried to recruit others for terrorist training and for terrorist acts and stated their “intention to be a suicide bomber,” prosecutors claimed.

Ashik Ali, 26, from Balsall Heath, Birmingham, did not travel to Pakistan, but allegedly provided premises for planning the attacks and making explosives.

He is said to have learned about explosives and detonators and was allegedly involved in the purchase of components and chemicals and in making a home made explosive device.

He too allegedly tried to recruit others for training and terrorism and stated that he wanted to be a suicide bomber.