Former PM likens battle to struggle against communism and warns that cost of standing aside would be far greater

The west's fight against al-Qaida is like the battle against revolutionary communism, says Tony Blair, who warns that it could last for a generation.

The former prime minister said on Sunday that Britain was right to send troops to support the French effort in Mali to put down a terrorist attempt to overthrow the country's government.

David Cameron faced difficult decisions to fight terrorism, Blair said, but warned the cost of standing aside would be far greater.

Britain at least had to try to "shape" events in the Middle East, he added, telling the BBC's Andrew Marr Show that in Syria there was already a danger the more extreme elements of the opposition forces fighting President Bashar al-Assad's regime would take over.

Blair said: "I think we should acknowledge how difficult these decisions are. Sometimes in politics you come across a decision which the choice is very binary, you go this way or that way and whichever way you go the choice is very messy.

"If we engage with this, not just militarily but over a long period of time, in trying to help these countries, it is going to be very, very hard but I think personally the choice of disengaging is going to be even greater."

He added: "We always want in the west, quite naturally, to go in and go out, and think there is a clean result. It's not going to happen like that. We now know that. It is going to be long and difficult and messy.

"My point is very simple though: if you don't intervene and let it happen, it is also going to be long, difficult and messy, and possibly a lot worse. It's a very difficult decision.

"We are certainly talking about a generation. I think a better way to look at it is like the fight the west had over a long period of time with revolutionary communism.

"It will happen in many different theatres, it will happen in many different ways but the truth is that you have no option but to confront it, to try over time to defeat it."