The Muslim prison population has leapt 50 per cent in the past decade.
But a lack of data means the jump in Muslim prisoners behind bars to 13,200 is unexplained, according to Labour MP David Lammy.
And experts have said radicalisation could not be the reason for such a large surge in Muslim prisoners.
Muslims now account for 15 per cent of all prisoners but five percent of the British population.
A report claims the increase is a ‘worrying’ trend and potential source of future ‘social division’.
Lammy said more data was needed to understand why the rise had occurred.
‘The lack of transparency undermines accountability,’ he said.
He added that that courts do not record defendants’ religion and said this practice meant it made it impossible to understand why the Muslim population behind bars had surged.
The report, published on Friday, was commissioned by David Cameron last year when he was Prime Minister.
It concludes: ‘We know far too little about what has been driving this trend. Are charging decisions, or trial outcomes affecting the numbers ending up in prison?
“Are large proportions of prisoners converting to Islam once they are in custody? We simply do not know. This gap needs to be taken seriously.”
One reason put forward for the increase is terrorism, but experts claim the relatively small conviction rate for such crimes cannot attribute for the rise.
And even the number of prisoners who have converted is too low, to account for the jump.
Lammy’s report did however find large racial disparities in sentencing.
His study found that for every 100 white women given prison sentences at crown courts for drug offences, 227 black women were given the same, reports HuffPost UK.
Similarly, For black men, it was 141 for every 100 white men.
Justice Secretary and Lord Chancellor David Lidington said figures about to be published by the government may shed some light on what is behind the rise.
‘We will always seek to drive out discrimination wherever it exists,’ he said.
‘That is why the Prime Minister ordered the unprecedented collection of data allowing the clearest understanding to date of how someone’s ethnicity can impact their day to day life, which will be published next month.’