The government has announced a series of measures intended to protect the travelling public from the emerging threat of cargo-hold bombs created by al-Qaida as questions continued to be asked about the initial British response to the alert.

Theresa May, the home secretary, outlined pre-departure checks that will be imposed on visitors seeking to enter and leave the UK, including electronic "no fly" lists for terrorism suspects, compiled as a result of passenger profiling.

As the government prepared to meet senior aviation figures to discuss the measures, German security officials disclosed that the two bombs, discovered at East Midlands airport and in Dubai on Friday morning, contained 300 and 400 grammes of the plastic explosive PETN – enough to put both aircraft at risk.

The devices had been built into desktop computer printers and are reported to have been wired to the printed circuit boards of mobile telephones.

In Washington, the US government was considering whether to grant the CIA far greater powers to select targets in Yemen for assassination by missiles fired from unmanned drones, despite mounting hostility in the country to such air strikes. Investigators from the US and the UK were travelling to the Yemeni capital Sana'a today in an attempt to identify those behind the bombs. Attention has focused on al-Qaida in the Arabian Peninsula and, in particular, upon a Saudi-born militant, Ibrahim al-Asiri, who is suspected of constructing the bombs.

He is also alleged to have been responsible for making the device involved in the failed Christmas Day bomb plot targeting a plane heading for Detroit last year.

The government was grappling with fears of a security loophole over freight that is heightened by the difficulty of detecting the explosive PETN, found in the printer bombs, but experts said it was detectable by scanners already in use in the UK to scan checked-in baggage. Extending the use of these scanners to cargo would, however, be complex and expensive, it was warned.

David Cameron told MPs that the UK must take every possible step to "cut out the terrorist cancer" that exists in the region, warning that the threat from Yemen had increased and that it was in the interests of the world to "come together to deal with this".

May said the government would step up checks on airline passengers seeking to enter the country "to identify better the people who pose a terrorist threat and to prevent them flying to the UK".

She also announced that from midnight the government was banning all unaccompanied air freight from Somalia. Similar steps were taken at the weekend over unaccompanied air freight from Yemen. The decision was taken because of "possible contact between al-Qaida in Yemen and terrorist groups in Somalia, as well as concern about airport security in Mogadishu", May told MPs.

For the next month, the government is also suspending the carriage of printer toner cartridges heavier than 500g in passengers' hand baggage on flights departing from UK airports, or their shipping as cargo unless they are handled by a company with government-approved security arrangements.

But tonight an inquiry was under way in Whitehall about why the prime minister's senior security and intelligence advisers did not tell him sooner about the attempted attack. It emerged that Cameron and May were not informed that the devices had been discovered until about 2pm on Friday, more than 10 hours after President Obama had been told.

They were briefed only when police were certain that the device found on a UPS cargo plane at East Midlands airport contained explosive material. The device was then dispatched to the Defence Science and Technology Laboratory at Fort Halstead in Kent. Examination by forensic scientists there led May to announce early on Saturday afternoon that the bomb was a "viable device".

One explanation offered tonight about the delay in telling May and Cameron was that the intelligence agencies receive so many alerts – most of them false alarms – that officials preferred to wait to establish the seriousness of this tipoff from the Saudis.

The delay in informing Cameron might also be explained because British intelligence initially appeared to believe that the plot might be either a dry run or simply an attempt to provoke panic. These assumptions – or hopes – were shattered when scientists discovered how sophisticated and potentially devastating the bombs were.

Yet since knowledge of the potential threat would have come from senior intelligence sources, there was some questioning in government of why the police did not send knowledge up the line earlier.

There was also concern that the bombs were detected as a result of a tipoff from Saudi intelligence sources, rather than as a result of scanning.

Lord Carlile of Berriew, who reviews the government's anti-terrorism legislation, said he had been deeply concerned three years ago about the lack of screening of cargo flights, but had not highlighted this in his annual report because he feared alerting terrorists. "I thought it might damage national security," he said. "I am absolutely certain that this is something that has been looked at carefully. There must be a very detailed operation enhancing the screening of cargo of whatever kind."

Andy Hayman, the former head of special operations at Scotland Yard, said there had been a "stuttering" police operation at the airport. Airline and security experts cautioned today that closing loopholes around freight would involve considerable cost and logistical complexity, requiring extending the use of sophisticated scanners used currently on passenger cargo to bulkier freight cargo.

Michael O'Leary, the boss of Ryanair, said a new security crackdown would be "pandering" to terrorists and mean more costly security measures for the industry. "They are laughing away in their caves this morning at the prime minister and his security team meeting to discuss printer cartridges," said O'Leary.

Reporting team: Alan Travis, Richard Norton-Taylor, Patrick Wintour, Chris McGreal in Washington, and Dan Milmo