David Cameron would be guilty of embarking on a “high-risk and pointless endeavour” if he sought to extend the British bombing mission against Islamic State forces in Iraq to Syria, a senior Tory MP and a former UK ambassador to Syria have warned.

As Downing Street lays the ground for a possible Commons vote to endorse an extension of the military campaign, Julian Lewis and Peter Ford said that joining forces with the US and other allied nations in the skies over Syria would be “extremely dangerous”.

Lewis, the Tory chairman of the House of Commons defence select committee, and Ford, a Labour member who served as ambassador to Syria between 2003 and 2006, issued their warning in a joint Guardian article.

The article says: “We have our differences, but both agree that bombing Syria under present circumstances would be a high-risk and pointless endeavour which should be rejected by the Commons, if the prime minister unwisely seeks to put it to the vote.”

Cameron is hoping to win parliamentary approval to extend RAF strikes against Isis targets in Iraq to Syria because ministers believe it is illogical to focus on one country and not the other.

The shadow chancellor, John McDonnell, has suggested that Jeremy Corbyn, who has said that he will oppose an extension of the airstrikes, will offer Labour MPs a free vote. The Tory whips are testing the ground to see whether they can win the support of enough Labour MPs to compensate for Lewis and up to 20 other Conservative MPs who will vote against the prime minister.

Lewis and Ford accuse Cameron of failing to provide convincing reasons for an extension of the airstrikes. They add that, unlike in Iraq, there are no credible ground troops in Syria to support an aerial bombardment.

The pair also raise concerns that Britain and its allies want to embark on “deliberate mission creep” in which they end up extending the strikes against Isis to target the regime of Bashar al-Assad.

The overthrow of Assad would lead to a victory for the Islamists – the precise opposite of the prime minister’s stated intention – Lewis and Ford say. They cite the example of the Anglo-French bombing of Libya which was originally designed to enforce a no-fly zone over Benghazi to protect its citizens but ended up creating the conditions on the ground for the overthrow of Muammar Gaddafi.

“Can we be sure that the prime minister would not seek opportunities to extend the bombing to the Syrian army as well as Isis?” they ask. “After all, we are constantly told that parliament was wrong to prevent the bombing of the Syrian army in 2013, and that it still remains essential to remove Assad.

“The government does not accept that its preferred ‘moderate’ forces are a fantasy and that a jihadi victory would be the only outcome if Assad were overthrown – with all the biblical-scale horrors which would flow from that for the Christians, Alawites, Shia and other minorities, as well as secular Sunnis.”

Ford and Lewis also warn that RAF pilots would be placed in severe danger, involving possible “dogfights” with Syrian aircraft, if they were obliged to protect safe havens. “This would result in enormous risks for the RAF,” they say, “flying in the same skies as Russian aircraft whose mission is to ensure the victory of the Syrian army.”