Ministers have consistently denied that jobcentres are being set targets for benefit sanctions in order to drive down welfare spending. On Tuesday, employment minister Mark Hoban told MPs: "There are no league tables in place. We do not set targets for sanctions."

But today's Guardian provides the clearest evidence yet that the practice has become endemic in the benefits system. In a leaked email, Ruth King, a manager at a Walthamstow jobcentre, is shown warning staff that they will be disciplined unless they increase the number of claimants referred to the "Stricter Benefits Regime", which could mean losing their payments for six months. Noting that Walthamstow is "95th in the league table out of only 109", she writes: "Guys, we really need to up the game here. The 5% target is one thing – the fact that we are seeing over 300 people a week and only submitting six of them for possible doubts is simply not quite credible."

Her advice to staff includes: "Do not accept the same job search every week, do not accept 'I dropped off CV to shops like Asda or Sainsbury's', listen for telltale phrases 'I pick up the kids', 'I look after my neighbours children/my grandchildren' or just 'I am busy' – all of which suggest that the customer may not be fully available for work, even cases where a parent shares custody can be considered."

Iain Duncan Smith has today responded by telling MPs that "There are no targets, there will be no targets and anybody caught imposing a target will themselves be dealt with." But with all evidence suggesting that the reverse is true, Labour is seeking to call the Work and Pensions Secretary to account. In a statement issued last night, Liam Byrne, Duncan Smith's shadow, said:

This explosive letter lays bare the climate of fear in Job Centres as league tables and threats of disciplinary action are used to perpetrate a culture of sanctioning innocent people to hit targets. That is just plain wrong and must be stopped – now. I asked ministers to assure the House on Tuesday that there were no sanctions targets and no league tables. Either Ministers have no grip on their department or they misled Parliament. Either way they must now face the consequences.

He has now written to Duncan Smith asking him to guarantee that the independent review promised by the government "will get to the bottom of every sanction issued by a job centre where targets were in operation".

After Labour's much-criticised decision to abstain in this week's vote on the coalition's workfare bill (which saw 43 MPs defy the whip to vote against it), the row is an opportunity for Byrne to strengthen his position. The promise of an independent review into the sanctions regime was one of the arguments he made for not voting against the legislation. He declared last night: "This is why we took difficult decisions on the Jobseekers' Bill to secure an independent review of sanctions. We knew there were sanctions targets and now we've secured an independent report to Parliament to put right a regime in Job Centres that's running out of control."