Tories should be careful what they wish for as Boris returns to parliament

Boris Johnson confirmed today he will seek to stand as an MP next year before finishing what will be his last term as London mayor.

"In all probability I will try to find somewhere to stand in 2015," he told journalists this morning, before adding: "I will serve out my mandate in London."

The announcement brings an end to years of speculation about his plans.

As reported yesterday, those close to the mayor have been desperately trying to convince him to stand for a third term as London mayor. However, the draw of making a bid for the Conservative leadership appears to have been too tempting for Johnson.

The announcement will be a huge relief to Labour, as they have struggled to come up with any candidates likely to beat Johnson in a mayoral election. It also means the end of his term in City Hall is now finally in sight.

Johnson's time as mayor has been electorally successful but largely unremarkable.

His biggest success, a central London bike hire scheme, is used by just a tiny percentage of Londoners. His other big ambitions, such as an estuary airport and an underground motorway, look likely never to see the light of day.

Arguably his greatest achievement was winning two elections against Ken Livingstone, who until Johnson showed up enjoyed high approval ratings among Londoners.

However, he has struggled to emerge from Livingstone's shadow at City Hall. Those hoping that he would radically alter Ken's agenda for London have been largely disappointed.

City Hall and Transport for London look much the same after six years of Johnson as they did under Livingstone. TfL, which the Tories fiercely attacked as "bloated" and wasteful in opposition, is almost unchanged and even has the same commissioner, in Peter Hendy, which it had under Livingstone.

And while Johnson entered City Hall declaring himself an "instinctive libertarian" he has become increasingly authoritarian in his instincts, culminating in his baffling decision to buy two second-hand water cannon from the German police.

Johnson has not changed London a great deal, but London has certainly changed him. When Johnson first declared his intention to stand for mayor, most commentators declared it a huge joke. Six years later and he is now being seriously considered as a future Conservative leader and even prime minister.

However, the Conservatives should be careful what they wish for. Johnson's time in City Hall has shown him to be a man with few if any set political principles. Fervently pro-immigration in front of business audiences, Johnson quickly becomes passionately sceptical in front of Conservative audiences. And while he today delivered a fiercely Eurosceptic speech, Johnson has not thought twice about relying on help from the EU when it has suited him.

It is also hard to imagine how Johnson would cope as leader of the opposition or prime minister. Widely viewed as amiable, Johnson has a fiery temper that few outside of City Hall have so far seen. Irritable under attack, he often becomes highly defensive and resorts to personal and often crude insults against his opponents on the London Assembly. While effective on panel shows and delivering off-the-cuff speeches, it is difficult to see how he would cope as leader against the combined jeers of hundreds of opposition MPs.

All of this will become clear if and when Johnson does return to the Commons.

But for now he can concentrate on what has become his defining specialist subject: the continued advancement of one Alexander Boris de Pfeffel Johnson.