Labour peer Lord Alli is being investigated by the House of Lords’ standards watchdog over allegedly failing to register interests.
A complaint was made during the last week about the long-standing Labour donor, who has been at the centre of a row over his donations of clothes to senior politicians, including Sir Keir Starmer.
The prime minister has insisted he has always followed the rules and said Labour politicians will no longer take donations like this.
As a peer in the House of Lords, Alli is required to register his interests in line with the parliamentary code of conduct.
A Labour spokesperson said: “Lord Alli will cooperate fully with the Lords’ Commissioner and he is confident all interests have been registered. We cannot comment further while this is ongoing.”
The BBC understands the investigation relates to a clerical element of already declared interests.
The specifics of the complaint have not been made public.
But Parliament’s website says Lord Alli is being investigated over potential breaches of rules about the “openness and accountability” of declarations.
The rules say peers must register “all relevant interests, in order to make clear what are the interests that might reasonably be thought to influence their parliamentary actions”.
The code of conduct also says: “Members are responsible for ensuring that their registered interests are accurate and up-to-date. They should register any change in their relevant interests within one month of the change.”
The complaint will be investigated by the Lords Commissioners for Standards, who are also investigating the former Tory peer Lady Mone over her alleged involvement for procuring PPE contracts during the pandemic.
The controversy over donations has dogged Sir Keir’s government since Labour won its landslide general election victory in July.
On Wednesday, Downing Street said the the prime minister had paid back more than £6,000 worth of gifts and hospitality received since the election.
The row over donations started in August, when it emerged that Lord Alli had been given a temporary Downing Street security pass despite having no formal government role. Labour has denied any wrong-doing.
Sir Keir has declared accepting donations from Lord Alli in the register of MPs’ interests, including accommodation worth more than £20,000.
The prime minister said this was to support his son during his GCSE exams.
Lord Alli has also made donations worth more than £32,000 for clothing and spectacles to Sir Keir.
Donations from Lord Alli were re-categorised in the register after Downing Street sought advice on declarations.
Sir Keir is one of seven cabinet ministers who received donations and gifts from Lord Alli in the lead-up to the general election.
Earlier this week, the government announced it would tighten the rules around ministerial hospitality to improve transparency.
Cabinet Office minister Pat McFadden told the BBC ministers would now have to declare hospitality linked to their government job in the register for MPs too.
The prime minister is not the only MP to have received freebies over the past year.
Many current MPs from all parties across the Commons list free tickets to sporting and cultural events in their registers of interests.
