Two men remain in custody over the Westminster attack as messaging services such as WhatsApp come under pressure to allow security services to eavesdrop on messages when a warrant is issued.

A 30-year-old man, arrested in Birmingham on Sunday evening on suspicion of preparation of terrorist acts, is the twelfth person to have been arrested since Khalid Masood killed four people and injured more than 50 others last week.

Image: Theresa May's handwritten card is among the many tributes outside Parliament

Nine people have been released without charge, while a 32-year-old woman from Manchester has been released on bail.

A 58-year-old man, arrested in Birmingham, remains in police custody after a total of 15 raids across the UK.

Meanwhile, Home Secretary Amber Rudd has criticised messaging services like WhatsApp for potentially allowing terrorists to communicate undetected.


Amber Rudd tackles tech giants

She called for encrypted networks to build "back doors" into their system so terrorists' messages can be accessed during investigations.

It came after it emerged that Masood sent an encrypted message via WhatsApp just minutes before his murderous assault.

:: Amber Rudd warns tech firms face 'ticking off' over terrorism

Memorials held for Westminster victims

Speaking to Sophy Ridge On Sunday, Ms Rudd also insisted social media companies must do more to deal with extremist material as part of the drive to combat terrorism.

She is due to meet with internet giants Google, Twitter, and Facebook later this week to discuss the issue, and warned: "They're going to get a lot more than a ticking off."

:: Westminster attack was over in just 82 seconds

Westminster attack was over in just 82s

In response WhatsApp said it had been assisting the police investigation.

A spokeswoman said: "We are horrified at the attack carried out in London earlier this week and are co-operating with law enforcement as they continue their investigations."

On Saturday police said they still believed Masood acted alone but said "there is a possibility we will never understand why he did this".