Britain must be prepared to cut a deal with Assad and Putin in the battle to defeat ISIS, Boris Johnson has warned.

The London Mayor said the West 'cannot afford to be picky about our allies' and it was 'obvious' that Syrian President Bashar al-Assad has ground troops in place who could take on the jihadist terror group.

It comes as polling shows a dramatic fall in the public appetite for British airstrikes against ISIS in Syria, with net support dropping by 31 points in just over a week.

London Mayor Boris Johnson said the West 'cannot afford to be picky about our allies' and it was 'obvious' that Syrian President Bashar al-Assad has ground troops in place who could take on the jihadist terror group

Britain has sent more fighter jets the RAF Akrotiri military base in Cyprus after MPs overwhelmingly backed airstrikes in Syria in a crunch vote last week.

At the weekend President Assad condemned the military action in his country as 'harmful and illegal', as he mocked David Cameron's 'classical farce' of claiming there were 70,000 moderate fighters in Syria ready to take on ISIS.

Today Mr Johnson suggested that the only way to take on ISIS on the ground was to rely on the forces under Assad's control - despite Mr Cameron wanting to bomb them two years ago.

The Syrian civil war has become increasing complex, with the Assad regime trying to face down a challenge from the Free Syrian Army while both are locked in bloody battles with ISIS militants operating across the Iraq-Syria border.

Writing in the Telegraph, Mr Johnson said: 'We need someone to provide the boots on the ground; and given that we are not going to be providing British ground forces – and the French and the Americans are just as reluctant – we cannot afford to be picky about our allies.'

He said the 70,000 fighters estimated by Mr Cameron may be 'exaggerated' and could include some 'jihadists who are not ideologically very different from al-Qaeda'.

Mr Johnson added: 'Who else is there? The answer is obvious. There is Assad, and his army; and the recent signs are that they are making some progress.

Who else is there? The answer is obvious. There is Assad, and his army; and the recent signs are that they are making some progress London Mayor Boris Johnson

'Thanks at least partly to Russian air strikes, it looks as if the regime is taking back large parts of Homs. Al-Qaeda-affiliated militants are withdrawing from some districts of the city. Is that a bad thing? I don’t think so.'

However, President Assad has so far shown little sign that he stands ready to work with the West.

He is unhappy that Britain did not ask for permission to launch airstrikes in his country, but Mr Cameron has insisted he can play no part in the long-term future of Syria after targeting his own people in deadly attacks.

President Assad condemned the involvement of Britain launching airstrikes in Syria.

'It will be harmful and illegal and it will support terrorism, as happened after the coalition started its operation a year or so [ago] because this is like a cancer,' he told The Sunday Times in an interview carried out on Wednesday.

'You cannot cut out part of the cancer. You have to extract it. This kind of operation is like cutting out part of the cancer. That will make it spread in the body faster.'

He added: 'You cannot defeat them [Isis] through airstrikes alone. You cannot defeat them without co-operation with forces on the ground.

'You cannot defeat them if you do not have buy-in from the general public and the government . . . . They are going to fail again.'

Attack: Extraordinary footage shows the moment British airplanes targeted ISIS oilfields in Syria, in the ongoing mission to destroy the terror group

Target: The strikes were the first time British Typhoon fighters have been deployed on bombing missions against ISIS in Syria

MPs last week voted overwhelmingly in favour of Britain extending military action to launch airstrikes against ISIS in Syria.

Within 57 minutes of the vote, RAF Tornado GR4 aircraft flew their first offensive operation against ISIS inside Syria, carrying out strikes against six targets in the extensive oilfield at Omar.

On Thursday night, Tornados conducted missions over Syria and Iraq, taking out a terrorist sniper team in Iraq with a precision Paveway IV guided bomb.

Then on Friday night, Tornado GR4s and Typhoon FGR4s again targeted Omar oilfields, using Paveway guided bombs to hit wellheads.

The RAF will start flying five missions a day – up from two previously – with two warplanes involved in each, six days a week.

However, public support for the mission has dropped sharply, according to polling by YouGov.

On November 24 - just over a week after the Paris terror attacks - 59 per cent of people approved of RAF strikes in Syria, while 20 per cent disapproved, putting net support on +39.

However, a survey conducted at the end of last week showed that only 44 per cent were in still in favour while 36 per cent were against, with net support on only +8.

Polling shows a dramatic fall in the public appetite for British airstrikes against ISIS in Syria, with net support dropping by 31 points in just over a week

YouGov's Will Dahlgreen said: 'Labour voters have had the most noticeable change of heart, with net support falling fully 48 points since November 24, but although a majority of UKIP voters are still in favour of air strikes net support among them has suffered a 36 point decline.

'The gap between men and women has narrowed slightly, but remains large – 52 per cent of men support air strikes on ISIS in Syria, while only 35 per cent of women feel the same way.'

Initial strikes have been against the Omar oilfields, but and many of the airstrikes will be carried out at night to try to limit the risk of civilian casualties.

But Defence Secretary Michael Fallon warned they could not be avoided altogether. 'War is a messy business; you cannot eliminate all risk,' he told the Sunday Times.

'It is very important for the campaign that we avoid mistakes. You can't completely eliminate mistakes in wartime, but we do our best to ensure that any civilian damage is minimised.