Sir Craig Oliver claimed the narrative in Westminster was Mrs May was going to leave after Brexit negotiations and the mixed message of announcing she is in it for the “long term” could cause problems.
The former Downing Street Director of Communications told BBC Newsnight: “The problem for Theresa May is that she has basically had a narrative in Westminster for the last few months that she is going to go after the Brexit negotiations.
“It’s actually what has been assumed in the Conservative Party and the thing about being so assertive now is that lots of people are going to say ‘hold on a minute I thought you were going to go, I thought we weren’t going to be in a position where you’re going to fight in the next Election’ and they are probably going to say that is a problem for them.”
He added: “No Prime Minister ever wants to get into a situation where they are setting a date for their departure and my old boss David Cameron discovered that to his cost 2015 here he said he wouldn’t serve a full term.”
Sir Craig comments come after a Tory MP told the Mirror that Mrs May would be stepping down from her post after Brexit, setting the date as August 30, 2019.
The alleged deadline would see her lead for five months after Britain leaves the EU on 29 March 2019, the nation’s official departure date.
But Mrs May has hit back at the rumours, saying she is in it for the long term.
Speaking to the BBC on Wednesday, the Prime Minister said: “Yes, there’s been an awful lot of speculation about my future which has no basis in it whatsoever.
Theresa May risks having a “lame duck premiership” as Prime Minister warns Sir Craig Oliver
“I’m in this for the long term.
“There’s a real job to be done in the United Kingdom.”
Peter Bone, also speaking on Newsnight, insisted Mrs May will deliver Brexit as he dismissed claims there were plots within the Tory party to remove her as Prime Minister.
The MP for Wellingborough and Rushden said: “I was amazed that how the one thing that she was fired up about and the one thing that was definitely going to happen is Brexit.
“And her eyes sparkled at the thought of it.
The problem for Theresa May is that she has basically had a narrative in Westminster for the last few months that she is going to go after the Brexit negotiations
“I thought wow this is just what we need and if there was anyone that had any doubt they should have been a that meeting.”
He added: “If she delivered Brexit, and I think she will, she will be a national hero.
“And then why not carry on? I have not heard a single person of all my backbench colleagues saying we have to get rid of the Prime Minister, we have to have a new leader, it has to be he or she.
“It just isn’t there. There is no plotting. By the way I think if was plotting going on I would probably know about it.”
According to reports Tory party members have thrown their support behind Brexiteer figureheads – including Brexit minister David Davis – and eurosceptic rising star Jacob Rees-Mogg.
Theresa May says she will stay on as Prime Minister – declaring she is in it for the long term
Last week bookmakers slashed the odds on Mr Rees-Mogg becoming the next Tory leader, as his supporters continue to rally behind the ‘Moggmentum’ campaign.
The MP is the 5/1 second favourite to replace Mrs May with bookies William Hill, after being 33/1 just one month ago.
The eccentric backbench MP for North East Somerset, who is pushing for a hard Brexit, is just behind Cabinet minister David Davis at 10/3.
Chancellor Philip Hammond is tipped at 7/1 to take over from Mrs May, while Foreign Secretary Boris Johnson is fourth favourite at 8/1.
Theresa May delivering Brexit will make her a national hero, Conservative MP Peter Bone declares
Mr Johnson in June revealed he will not challenge the Theresa May for Prime Minister until after Brexit – when rumours began circulating of a possible leadership contest.
When asked whether Mrs May will remain Prime Minister, Mr Johnson told Channel 4: “What I tell you is that Theresa May got more votes than anybody since Margaret Thatcher in 1983.
“She leads the largest party in Parliament by a margin of 56 seats. It is her duty to go on and what she can.
“I think it was an excellent Queen’s Speech.”