At least three terror groups plotting attacks against Britain have changed their communication methods since the leaks by Edward Snowden, it was claimed last night.

The Al Qaeda-linked networks have altered their tactics since the fugitive stole intelligence files from GCHQ and the US National Security Agency, according to a report.

Extremist websites have also moved to protect their digital communications by releasing encryption programmes for followers, making it harder for extremists to be tracked down.

Scroll down for video

Last Week Tonight host John Oliver flew to Moscow to grill Edward Snowden about the leaking of 'harmful' documents and asks him to explain why he did it

Evidence of the harm done to intelligence agencies across the world after the former CIA contractor leaked 1.7million classified documents in 2013 is compiled in the first comprehensive public analysis of the leaks by the Henry Jackson Society security think-tank.

The Snowden files revealed details about covert attempts by agencies to view citizens' private information by gathering internet histories, emails, text and call records and passwords.

Now the Henry Jackson Society has found that at least three Al Qaeda affiliates are known to have altered their communications. The think-tank has also found other groups using human couriers instead of email and mobile phones.

Snowden fled justice in the US to Hong Kong, then Russia, where he was granted asylum and now lives in a secret location

Jihadist websites also released three 'significant' encryption programmes 'within a three- to five-month time frame of the leaks', meaning fanatics are harder to detect and encrypted emails take longer to decipher.

Report author Robin Simcox said: 'The focus has been about the so-called mass surveillance by GCHQ and the NSA but these leaks have allowed extremist groups and jihadists to look at our capabilities – what we can and can't do – which has given them a great insight.

'At a time when the range of threats against the West has never been greater, with Yemen, Iraq, Syria, Somalia and Al Qaeda in Pakistan, it is astonishing the focus has been more on the shortcomings of our intelligence agencies and not the fact Snowden has helped terror suspects drop off the radar.

'He has caused severe damage to our ability to fight extremism.'

MI5 director general Andrew Parker has called the traitor's actions a 'gift to terrorists'.

On Monday, Snowden revealed he had not read all the top-secret dossiers he stole before handing them to newspapers including the Guardian – a move which has put lives in jeopardy from terrorists.

He claims he had to act because the US and other Western governments' policies were a 'threat to democracy'.