A 30-year-old man questioned by counter-terrorism police investigating a sword attack on police officers outside Buckingham Palace has been released without charge.

Officers still have until Friday to question a 26-year-old man, from Luton, who was arrested at the scene on suspicion of terror offences, grievous bodily harm and assault on police, Scotland Yard said.

Three officers suffered minor injuries after a suspect deliberately drove a car at police outside Buckingham Palace and then reached for a 4ft sword shortly after 8.30pm on Friday.

The man, who drove a blue Toyota Prius at a marked police vehicle, repeatedly shouted "Allahu Akbar" before he was incapacitated with CS spray.

Officers now believe the suspect was acting alone and the second man, arrested in west London on Sunday, will face no further action.

Commander Dean Haydon, the head of the Met's Counter Terrorism Command, praised the quick action of the officers, two of whom needed hospital treatment for cuts.

"I would like to pay tribute to the bravery and professionalism of these officers who quickly brought this incident under control," he said.

"Their vigilance, courage and the swiftness of their response demonstrates how our officers are protecting the public at this time.

"Officers from the Counter Terrorism Command are now investigating and searches have been carried out in the Luton area.

"We believe the man was acting alone and we are not looking for other suspects at this stage. While we cannot speculate on what the man was intending to do - this will be determined during the course of the investigation - it is only right that we investigate this as a terrorist incident at this time.

"I would urge anybody who witnessed the incident or has any information or images that may assist the investigation to contact the confidential hotline on 0800 789 321."

It is understood that no members of the Royal Family were in Buckingham Palace when the attack happened in the restricted area in Constitution Hill.

Mr Haydon urged the public to be alert and report any suspicions to police.

He added: "This is a timely reminder that the threat from terrorism in the UK remains severe. The police, together with the security services, are doing everything we can to protect the public."