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The elections watchdog is to launch an investigation into polling in Tower Hamlets as the borough was finally due to announce its results tonight.

Five full days after voting closed, the council was today still to confirm its full list of councillors after the controversial mayor, Lutfur Rahman, edged back into power.

Recounts and a high turnout in the borough even triggered delays in the announcement of London's European election results on Sunday night.

Labour are neck-and-neck with Mr Rahman’s newly-formed Tower Hamlets First party in the battle to form a majority in the council chamber, with 18 seats each and two wards still up for grabs.

The Electoral Commission has now said it will look into "issues" surrounding the poll after complaints of voter intimidation and large discrepancies in recounts.

A spokesman said: "Clearly there have been issues at the Tower Hamlets count and we need to make sure we understand what happened, and the reasons for it, before reaching any conclusions.

“As part of our review we will be talking to the Returning Officer and Regional Returning Officer. We will be looking closely at what happened during the count, as well as the planning that took place beforehand.

“Once we have concluded the review we will publish its findings in our report on the elections."

Labour are understood to be resigned to the prospect of councillors jumping ship to work with Mr Lutfur Rahman after his win over John Biggs in the the mayor election.

A senior Labour source said it was a “racing certainty” that councillors would cross the aisle to work with Mr Rahman – whoever wins in the Bromley South ward when it is announced tonight.

The source said: “Are we worried? Of course, but maybe more resigned to the political realities of Tower Hamlets. We’ll obviously do what we can to avoid it.”

Mr Rahman is seeking to form his first majority in the chamber since he first swept to power in 2010.

To do so, he would either need to win both seats in Bromley South and bag a clean sweep in a forthcoming by-election after a candidate's death in Blackwall and Cubitt Town, or else tempt Labour councillors into defecting.

It is understood there have been at least two recounts in Bromley South - with claims of a discrepancy of as much as 21 per cent in one.

After triumphing on a chaotic vote count night in the art deco Troxy theatre in Limehouse, Mr Rahman made a direct appeal to his rivals to work with him.

"The last 6 years of Tower Hamlets politics have been acrimonious," he said. "There is a clear majority, but it is also clear that there is a need for reconciliation.

"I have already said that I will work with John Biggs and I again extend my hand to the local Labour Party – let’s work together for the whole community."

The by-election in Blackwall and Cubitt Town is due to be held in the next four weeks after a candidate died suddenly the day before voters went to the polls.